Move root documentation into docs tree

[aaltshuler]

Summary

• move the tracked RFC-026 supporting notes from the repository root into docs/dev
• normalize WAL-thinking.md to lowercase kebab-case and repair all relative links
• index the supporting analysis from the developer docs entry point
• place the v0.8.1 community announcement alongside release notes
• keep conventional repository discovery and governance documents at the root

Why

The repository root should contain project entry points and control files. Design analysis belongs in the developer documentation taxonomy, while release-specific communication belongs with release history.

Validation

• scripts/check-agents-md.sh
• git diff --check
• verified all moved-document relative targets exist

Dependency

Stacked on #379 because two of the moved WAL documents are introduced there. Retarget this PR to main after #379 merges.

Greptile Summary

This PR organizes design and release documentation under the existing docs tree. The main changes are:

• Moves RFC-026 supporting notes into docs/dev.
• Renames the WAL notes to lowercase kebab-case.
• Repairs relative links in the moved documents.
• Adds the supporting analysis to the developer docs index.
• Places the v0.8.1 community announcement under docs/releases.

Confidence Score: 5/5

This looks safe to merge.

• The previously broken Slack-style links now use standard Markdown syntax.
• No incomplete or unsafe fix remains in the changed documentation.

Important Files Changed

Filename	Overview
docs/dev/index.md	Adds an index entry for the relocated RFC-026 supporting analysis.
docs/dev/lance-memwal-pr.md	Moves the Lance MemWAL proposal into developer docs and updates its relative links.
docs/dev/wal-options.md	Moves the WAL options analysis into developer docs and repairs internal links.
docs/dev/wal-thinking.md	Moves and renames the WAL notes while updating links to RFCs and test files.
docs/releases/v0.8.1-community.md	Adds the v0.8.1 community announcement with standard Markdown links.

_{Reviews (2): Last reviewed commit: "Move root documentation into docs tree" | Re-trigger Greptile}

Context used:

• Context used - AGENTS.md (source)
• Context used - CLAUDE.md (source)