Reorganize docs into multiple pages

[melissamcewen]

Docs should be multiple pages rather than one long page.

• Reorganize file structure so it's easier to make multiple pages see [WIP] Initial migration of docs to Snowpack #1413
• Add new guides/guide section see PR React + Snowpack Guide #1309
• Reorganize content Reorganizing docs #1430
• Add new TOC /split into pages Docs split into multiple pages #1437
• Audit content/links to find broken links, see if we need to do some editing (check posts too)
• Merge any docs changes since docs-refactor branch was created
• Merge into main docs/deploy