Cleanup docs source filenames for Hugo migration (batch 1)

[Flamki]

Summary
Follow-up for #538: clean source filenames by removing section prefixes in selected docs sections.

Scope of this batch

• content/docs/fundamentals/*
• content/docs/running/*
• content/docs/tooling/*
• content/docs/docs-meta/*

What changed

• Renamed files from prefixed names (for example fundamentals-overview.md) to section-local names (for example overview.md).
• No content edits.
• No permalink/frontmatter changes.

Why this is safe

• Jekyll output paths remain unchanged because existing explicit permalinks are kept.
• This incrementally prepares Hugo-compatible naming without changing rendered URLs.

Validation

• pre-commit run --files <renamed files> (passed)
• docker run --rm -v "${PWD}:/srv/jekyll" -w /srv/jekyll jekyll/jekyll:4 bash -lc "bundle install && bundle exec jekyll build" (passed)

Relates to #538
Parent context: #472

[MakisH]

As I also wrote in #752 that also addresses this:

Thank you for the contribution. We can only review this later, as this is work intended for the framework migration project and capacity right now is limited.

Not sure right now if this is a good idea.

[precice-bot]

This pull request has been mentioned on preCICE Forum on Discourse. There might be relevant details there:

https://precice.discourse.group/t/gsoc-2026-website-modernization-interest-ayush-singh-flamki/2766/1