style: give buttons a clear hierarchy and mark external links (#50)

## What

On the Getting Started page, every call-to-action button was rendered as
a filled **primary** button. When everything is primary, nothing is —
the eye has no anchor and the page reads as a wall of equally-weighted
blue buttons.

This PR introduces a visual hierarchy and distinguishes links that leave
the docs site.

### Button hierarchy

Keep the filled `.md-button--primary` style on just **two** buttons —
the recommended entry point in each of the two decision areas:

- **GitHub Actions** — the recommended integration in *Choose Your
Integration*
- **clang-tools-pip** — the recommended installer in *Install the clang
tools*

Every other button is demoted to the outlined `.md-button` style, so
each section now has one obvious primary action and a set of
clearly-secondary alternatives.

### External-link marking

Buttons that navigate away from the docs site (to `github.com` /
`pypi.org`) now end with an external-link icon
(`:octicons-link-external-16:`) instead of the `→` arrow. The two
in-ecosystem `cpp-linter.github.io` links keep the `→`, signalling
"you're staying on the docs site."

## Why

- Clearer guidance: a first-time visitor is steered toward the two
recommended paths instead of facing a dozen equal choices.
- Honest affordances: users know before clicking whether a button keeps
them on the docs or sends them to GitHub/PyPI.

## Notes

Content, links, and card structure are unchanged — this is purely a
styling/affordance pass.

---
_Generated by [Claude
Code](https://claude.ai/code/session_012uXrN1wk5EaqK5GimN3deT)_

Author: shenxianpeng

docs/getting-started.md

@@ -44,7 +44,7 @@ Select the method that best fits your development workflow:
 
     **Perfect for:** Catching issues before commits, local enforcement
 
-    [Get started with pre-commit →](https://github.com/cpp-linter/cpp-linter-hooks){ .md-button .md-button--primary }
+    [Get started with pre-commit :octicons-link-external-16:](https://github.com/cpp-linter/cpp-linter-hooks){ .md-button }
 
 - :fontawesome-brands-python: **Command Line (Python)**
 
@@ -54,7 +54,7 @@ Select the method that best fits your development workflow:
 
     **Perfect for:** Local development, custom scripts, one-off analysis
 
-    [Get started with cpp-linter package →](https://pypi.org/project/cpp-linter/){ .md-button .md-button--primary }
+    [Get started with cpp-linter package :octicons-link-external-16:](https://pypi.org/project/cpp-linter/){ .md-button }
 
 - :simple-rust: **Command Line (Rust)**
 
@@ -64,7 +64,7 @@ Select the method that best fits your development workflow:
 
     **Perfect for:** Local development, custom scripts, one-off analysis
 
-    [Get started with cpp-linter-rs →](https://github.com/cpp-linter/cpp-linter-rs){ .md-button .md-button--primary }
+    [Get started with cpp-linter-rs :octicons-link-external-16:](https://github.com/cpp-linter/cpp-linter-rs){ .md-button }
 
 </div>
 
@@ -91,15 +91,15 @@ Choose whichever fits your setup.
 
     Install `clang-format`, `clang-tidy`, `clang-query`, `clang-apply-replacements`, and `clang-include-cleaner` via Homebrew — no compilation required
 
-    [Get started with Homebrew →](https://github.com/cpp-linter/homebrew-tap){ .md-button .md-button--primary }
+    [Get started with Homebrew :octicons-link-external-16:](https://github.com/cpp-linter/homebrew-tap){ .md-button }
 
 - :fontawesome-brands-python: **clang-tools-asdf**
 
     ---
 
     Easy installation of clang tools static binaries via asdf
 
-    [Get started with asdf →](https://github.com/cpp-linter/asdf-clang-tools){ .md-button .md-button--primary }
+    [Get started with asdf :octicons-link-external-16:](https://github.com/cpp-linter/asdf-clang-tools){ .md-button }
 
 </div>
 
@@ -113,23 +113,23 @@ Choose whichever fits your setup.
 
     Statically-linked `clang-format`, `clang-tidy`, `clang-query`, `clang-apply-replacements`, and `clang-include-cleaner` binaries for Linux, macOS, and Windows
 
-    [Download from →](https://github.com/cpp-linter/clang-tools-static-binaries){ .md-button .md-button--primary }
+    [Download from :octicons-link-external-16:](https://github.com/cpp-linter/clang-tools-static-binaries){ .md-button }
 
 - :fontawesome-brands-docker: **clang-tools-docker**
 
     ---
 
     Docker images with pre-installed `clang-format` and `clang-tidy`
 
-    [Download from →](https://github.com/cpp-linter/clang-tools-docker){ .md-button .md-button--primary }
+    [Download from :octicons-link-external-16:](https://github.com/cpp-linter/clang-tools-docker){ .md-button }
 
 - :fontawesome-brands-python: **clang-tools-wheels**
 
     ---
 
     Redistribute `clang-format` and `clang-tidy` Python wheels
 
-    [Download from →](https://github.com/cpp-linter/clang-tools-wheel){ .md-button .md-button--primary }
+    [Download from :octicons-link-external-16:](https://github.com/cpp-linter/clang-tools-wheel){ .md-button }
 
 </div>
 
@@ -143,14 +143,14 @@ Choose whichever fits your setup.
 
     Install `clang-apply-replacements` via pip — standalone Python wheel
 
-    [Install from PyPI →](https://pypi.org/project/clang-apply-replacements/){ .md-button .md-button--primary }
+    [Install from PyPI :octicons-link-external-16:](https://pypi.org/project/clang-apply-replacements/){ .md-button }
 
 - :fontawesome-brands-python: **clang-include-cleaner**
 
     ---
 
     Install `clang-include-cleaner` via pip — detects unused `#include` directives
 
-    [Install from PyPI →](https://pypi.org/project/clang-include-cleaner/){ .md-button .md-button--primary }
+    [Install from PyPI :octicons-link-external-16:](https://pypi.org/project/clang-include-cleaner/){ .md-button }
 
 </div>