docs: rename index.md to README.md in docs directories

[andyfeller]

Summary

Renames all index.md files under docs/ to README.md so that GitHub.com automatically renders them when browsing each directory.

Changes

• Renamed 8 files: docs/index.md, docs/auth/index.md, docs/features/index.md, docs/hooks/index.md, docs/integrations/index.md, docs/observability/index.md, docs/setup/index.md, docs/troubleshooting/index.md → each corresponding README.md
• Updated all internal cross-references in docs/**/*.md and root README.md to point to README.md instead of index.md

Motivation

GitHub renders README.md files automatically when you visit a directory on GitHub.com. Using index.md requires an explicit file click, which makes the docs less discoverable when browsing the repo.

[Copilot]

Pull request overview

This PR renames the docs/**/index.md landing pages to README.md so GitHub renders each docs section automatically when browsing directories, and updates internal links accordingly.

Changes:

• Updated root README.md and docs/**/*.md cross-references to point to the new docs/**/README.md landing pages.
• Added/updated section landing pages under docs/ (setup, auth, features, hooks, integrations, observability, troubleshooting).
• Adjusted “See also”/navigation links in several docs pages to match the new README-based structure.

Show a summary per file

File	Description
README.md	Updates doc links to target docs/**/README.md landing pages.
docs/README.md	Updates the main docs landing page to point at section README.md files.
docs/setup/README.md	Adds a setup section landing page for directory rendering.
docs/troubleshooting/README.md	Adds a troubleshooting section landing page for directory rendering.
docs/observability/README.md	Adds an observability section landing page for directory rendering.
docs/integrations/README.md	Adds an integrations section landing page for directory rendering.
docs/hooks/README.md	Adds a hooks reference landing page for directory rendering.
docs/features/README.md	Updates feature index references to the new hooks landing page path.
docs/auth/README.md	Adds an authentication landing page for directory rendering.
docs/setup/multi-tenancy.md	Updates “See also” link to the features landing page path.
docs/hooks/user-prompt-submitted.md	Updates “See also” link to ./README.md.
docs/hooks/session-lifecycle.md	Updates “See also” link to ./README.md.
docs/hooks/pre-tool-use.md	Updates “See also” link to ./README.md.
docs/hooks/post-tool-use.md	Updates “See also” link to ./README.md.
docs/hooks/error-handling.md	Updates “See also” link to ./README.md.
docs/features/cloud-sessions.md	Updates authentication link to the new auth landing page path.
docs/auth/byok.md	Updates “Next steps” link to the new auth landing page path.

Copilot's findings

Comments suppressed due to low confidence (1)

docs/README.md:12

• This PR removes all docs/**/index.md files by renaming them to README.md. However, the repo’s docs style guide states that in the docs pipeline, index.md files are treated as YAML-only category pages for docs.github.com. If these index.md category pages are still required for the sync/navigation pipeline, renaming them away will likely break that pipeline. Consider keeping index.md (YAML-only) for the pipeline and adding README.md for GitHub directory rendering, or otherwise updating the pipeline to recognize README.md as the category page entrypoint.

• Files reviewed: 11/17 changed files
• Comments generated: 2