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?