Feat: site build enhancements

[espy]

This PR adds various improvements to the site, mainly concerning the build process, and removes, simplifies and hardens a bunch of build and dev server related stuff. These are in preparation for #9246, so that all gets easier to do.

Overview

• Uses eleventy-plugin-syntaxhighlight instead of custom highlighting code, adapts code.js to work with this new output.
• Restructures layouts and enforces full filenames with extensions when using includes (faster and less error-prone).
• Use a less roundabout way of compiling LESS files.
• Different handling of js and css minification (js via an 11ty transform, css via an npm cli command).
• Removes a bunch of pre- and post-processing from build-site.js and the 11ty config that don't seem to be needed any longer.
• Use execa instead of exec to run CLI scripts in build-site, this now correctly displays logs from 11ty, handles cleanup of orphaned child processes properly, and resumes work after an 11ty crash and subsequent rebuild without having to restart the process manually.
• Removes the stickyfill polyfill, as position: sticky is widely available.
• Reformats the serviceworker output and adds a line to stop SW from intercepting browser extension requests, and another to only intercept GET requests. This broke the use of the PouchDB instance in the browser console, for example when demoing replication to a local CouchDB, since the SW would intercept the POST requests and then fail, because it can’t cache them.
• Fixed the github edit button linking to the wrong place occasionally.
• Various other smaller CSS improvements and fixes to the site content.

Testing recommendations

Build the site locally and compare it to pouchdb.com:

$ npm i
$ npm run build-site

Checklist

• I am not a bot
• This is my own work, I did not use AI, LLM's or similar technology for code or docs generation
• Code is written and works correctly