Unify generation of Scala 3 API and Reference documentations

[WojciechMazur]

This PR does unify how snapshot and stable scaladoc artifacts are published:

• docs/sidebar.yml now contains only entries visible for stable releases
• docs/sidebar.nightly.template.yml contains entries specific for Snapshot builds (contributing guide, blog posted at https://dotty.epfl.ch/) - it does not contain reference subsection entries - this part is now generated when executing generateScalaDocumentation task. It emits a patch to docs/sidebar.yml containg combined inputs from both files
• project/resources/referenceReplacements/_layouts/static-site-main.html was removed, instead we now inject the warning only when building snapshot docs using generateScalaDocumentation
• project/resources/referenceReplacements/sidebar.yml is removed, we would no longer need to keep 2 files in sync
• generateStableScala3Documentation no longer uses calculated paths to obtain compilation outputs - it was only needed in the past to allow for generation of new scaladoc for older compiler/library

Fixes #22154 - or rather introduces fix in the main repo. During the releases we're applying most of these patches manually to workaround the issue

[WojciechMazur]

This file is exactly old sidebar.yml with reference sections removed (+ additional comment)

[WojciechMazur]

This file is exactly the old sidebar.yml for only reference related sections.
Additions here are mostly due to diverging state of https://github.com/scala/scala3/tree/language-reference-stable that was not sync with main

[WojciechMazur]

We'd like to merge it after series of ongoing changes to building Scala 2 stdlib in our build, discussed with @hamzaremmal about it.

[hamzaremmal]

This one (#23744) is going through today (probably). It might affect the work here.

[WojciechMazur]

Rebased (and squashed) onto the current main. The second main restores configuration for using markdown syntax and snippet compiler (test locally)

This change:

Vs snapshot (apparently for version 3.8.0-RC1-bin-20250822-658c8bd-NIGHTLY found on https://dotty.epfl.ch/, https://nightly.scala-lang.org/ has 3 days older version. Maybe publishing nightly docs is broken?

[tgodzik]

This will need a separate PR to the LTS branch @WojciechMazur

[hamzaremmal]

Why even backport it when we don't release nightly documentation for the LTS? The old system works there and can still be used for the next year and a half.

[WojciechMazur]

Mostly to cleanup the docs when doing the release, otherwise each time we need to manually remove nightly warning, blogs, and contributing sections. So in fact for LTS we can just remove these without doing a full backport

[hamzaremmal]

Mostly to cleanup the docs when doing the release, otherwise each time we need to manually remove nightly warning, blogs, and contributing sections. So in fact for LTS we can just remove these without doing a full backport

Then it is a different thing. not a backport.