Added General navigation section.
The General navigation section now contains:
- Meta (moved from Overview)
- Rate Limit (moved from Overview)
- Gitignore (moved from its own top-level nav section)
- Markdown (moved from its own top-level nav section)

All existing URLs remain unchanged. The /v3/general URL has been
added as a summary page for the General navigation section.
Adds documentation for the /emojis endpoint.
Added Emojis section to /v3/general summary page.
Alphanumeric ordering :sparkles: for General navigation section and summary page.
Link to detailed API docs from summary page.
Use Miscellaneous rather than General. Sort top-level nav items.
Merge pull request #302 from github/add-general-api-nav-section

Add Miscellaneous API navigation section.