Add visualization and renderer documentation

[jswir]

1. moved renderer docs into official malloy docs
2. Updated about_rendering.malloynb with improved structure
3. Add renderer_tags.malloynb with comprehensive tag reference
4. Add renderer_cheatsheet.malloynb with quick reference table
5. Add plugins.malloynb for custom renderer plugins
6. Update table of contents with new visualization pages

Issues to resolve:

• renderer v282 didn't show multi-line chart and upgrade to v324 cause server-side rendering bugs.
• some tags appear to not work - e.g., # pivot and # tranpose