#23762: docs: fix markdown formatting and structure issues

## Summary - **VISION.md**: Use H1 for document title; add blank line and bullet list for cross-reference links - **CONTRIBUTING.md**: Rename duplicate `## Maintainers` heading to `## Becoming a Maintainer` - **grammy.md**: Demote extra H1 headings to H2; add missing `##` prefix on "Open questions" - **bluebubbles.md**: Consolidate duplicate "Typing indicators" bullet into one - **platforms/index.md**: Remove redundant sentence about Windows companion apps ## Test plan - [x] Verify docs render correctly on Mintlify <!-- greptile_comment --> <h3>Greptile Summary</h3> This PR fixes markdown formatting and structure issues across 5 documentation files. All changes are formatting-only and improve document structure: - `VISION.md`: Changed document title from H2 to H1 (`#` instead of `##`), added blank line for better separation, converted cross-reference links to a proper bullet list - `CONTRIBUTING.md`: Renamed duplicate "Maintainers" heading to "Becoming a Maintainer" to distinguish it from the existing maintainer list section - `grammy.md`: Demoted extra H1 headings to H2 for proper hierarchy ("Why grammY", "What we shipped"), added missing `##` prefix to "Open questions" - `bluebubbles.md`: Consolidated duplicate "Typing indicators" bullet points into a single comprehensive entry - `platforms/index.md`: Removed redundant sentence about Windows companion apps (was mentioned twice with slightly different wording) All changes follow proper markdown conventions and improve documentation clarity without altering any technical content. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with no risk - All changes are purely cosmetic markdown formatting fixes with no impact on functionality, logic, or behavior. The changes improve document structure and readability by fixing heading levels, removing duplicates, and organizing content properly. No code or configuration is modified. - No files require special attention _{Last reviewed commit: e36cf3c} <!-- greptile_other_comments_section --> _{(4/5) You can add custom instructions or style guidelines for the agent [here](https://app.greptile.com/review/github)!} <!-- /greptile_comment -->