Add missing section _index.md files for Hugo migration prep

[Flamki]

Summary
Continue #538 by adding missing section _index.md files required for Hugo section recognition.

Context

• Restructure website content #580 restructured content hierarchically.
• Content files restructure for compatibility with Hugo #538 still listed a remaining TODO: section folders need _index.* files.
• Jekyll output remains unchanged because _config.yml excludes content/**/_index.md.

What this PR does

• Adds empty _index.md files to section folders under content/docs and content/community that contain markdown pages but had no section index file.
• Scope is intentionally narrow: no file renames and no permalink changes.

Validation

• Verified no markdown-bearing section folder remains without _index.md.
• pre-commit run --files on all newly added files.
• Docker Jekyll build completed successfully.

Relates to #538
Parent context: #472

[MakisH]

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 these files are needed and should be like this.

[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

[MakisH]

Since the content is changing as we are going towards the migration, I would rather close this to not intermix with the actual project. We will need to add content and test it anyway.