Merge into Firedancer monorepo

[riptl]

This specs site should be merged into the main Firedancer repo. Checking in docs improvements along with code changes is good practice, and allows us to benefit from the Gerrit review process.

Questions to resolve:

• Should we retain commit history or squash?
• How to merge: git subtree or unrelated-history merge with subtree strategy?
• Should docs (and supporting files like package.json) be at the monorepo root or in a /docs/ subdir?

[riptl]

cc @leoluk

[riptl]

I'm personally in favor of

• retaining history
• unrelated-history merge
• move everything to /docs/ to not clutter root with dotfiles and config