Switch to MkDocs and hosted documentation

[zanieb]

This is a tracking branch for moving from GitHub markdown documentation to MkDocs and hosted documentation on docs.astral.sh.

It includes the following changes:

• Copy basic template and styling for documentation from uv #729
• Build the documentation in CI #734
• Publish documentation on release #731
• Add favicon #746
• Add ty letter logo asset #747
• Naively split the "Overview" page into child pages #748
• Minor adjustments to docs #752

And some to-do tasks are:

• Update the GitHub admonitions to MkDocs formatting
• Organize the documentation into new sections
• Revisit generation of reference documentation (see Reference documentation #292)
  • Fix the rules reference generation (the <details> sections don't look good)
• Update the docs links in the repo-root README.md
• Update https://ty.dev/rules to point to the new rules-reference page, not to https://github.com/astral-sh/ty/blob/main/docs/reference/rules.md

Post-merge to-do tasks:

• Consider writing guides for core workflows
• Consider using pre-commit instead of the prettier format command (adapt CONTRIBUTING.md)

The following changes are part of this task but can be merged to main prior to this feature:

• Add documentation publishing workflow #730