refactor(www): use optional dynamic route params in the docs/[...slug].tsx route

[bhb603]

Resolves #1750

• refactor the docs/[...slug].tsx to use optional route params: docs/[[version]]/[...slug]
• cleaned up the docs/index.tsx handler as well

Testing

I believe this preserves current routing behavior:

• /docs/:version -> Redirects to getFirstPageUrl(version)
  • e.g.
    • http://localhost:8000/docs/latest -> 307 /docs/introduction
    • http://localhost:8000/docs/canary -> 307 /docs/canary/the-canary-version
• /docs/:slug -> renders latest docs
  • e.g.
    • http://localhost:8000/docs/introduction
    • http://localhost:8000/docs/getting-started
    • http://localhost:8000/docs/getting-started/create-a-project
    • http://localhost:8000/docs/concepts
    • http://localhost:8000/docs/concepts/routing
    • http://localhost:8000/docs/examples/writing-tests
• /docs/latest/:slug -> renders latest docs
  • e.g.
    • http://localhost:8000/docs/latest/introduction
    • http://localhost:8000/docs/latest/getting-started
    • http://localhost:8000/docs/latest/getting-started/create-a-project
    • http://localhost:8000/docs/latest/concepts
    • http://localhost:8000/docs/latest/concepts/routing
    • http://localhost:8000/docs/latest/examples/writing-tests
• /docs/canary/:slug -> renders canary docs
  • e.g.
    • http://localhost:8000/docs/canary/introduction
    • http://localhost:8000/docs/canary/getting-started
    • http://localhost:8000/docs/canary/getting-started/create-a-project
    • http://localhost:8000/docs/canary/concepts
    • http://localhost:8000/docs/canary/concepts/routing
    • http://localhost:8000/docs/canary/examples/writing-tests
• /docs/:invalid-slug -> 404
  • e.g.:
    • http://localhost:8000/docs/foo-bar
    • http://localhost:8000/docs/foo-bar/baz
• /docs/latest/:invalid-slug -> 404
  • e.g.:
    • http://localhost:8000/docs/latest/foo-bar
    • http://localhost:8000/docs/latest/foo-bar/baz
• /docs/canary/:invalid-slug -> 404
  • e.g.:
    • http://localhost:8000/docs/canary/foo-bar
    • http://localhost:8000/docs/canary/foo-bar/baz

[bhb603]

It was unclear to me how this handler could ever have ctx.params.slug, since it's not a dynamic route, so I removed.

But if there is another way and I'm mistaken, LMK and we can add this back!