Docs - Redirect dead docs URLs and fix in-repo links that point at them

[krisajenkins]

Description of Changes

Old links into the docs are broken in two ways, and this PR fixes both. Links from outside the repo get redirects; links inside the repo are corrected to point at the real page.

1. Redirect the docs URLs the Docusaurus migration left behind

Follow an old link into the docs (a blog post, a bookmark, a search result) and you get raw S3 XML (NoSuchKey), not even a 404 page. Two families of URL are dead:

• Legacy flat URLs from the pre-Docusaurus site (/docs/unity/part-1, /docs/sdks/rust, /docs/spacetimeauth/create-project, …), gone since the migration (Docusaurus migration #3343).
• Folder-shaped paths such as /docs/intro/tutorials/chat-app. Nearly every doc sets a slug that flattens its folder out of the URL, so the path you'd guess from the source tree 404s.

@docusaurus/plugin-client-redirects was already a dependency but never configured. This wires it up with docs/redirects.ts:

• a hand-mapped legacy list, taken from the old docs/nav.ts and cross-checked against the Internet Archive;
• folder-path → slug redirects generated from the docs tree, so new docs get them for free.

The S3 website endpoint serves directory indexes, so each generated …/index.html stub is reached and followed.

It also fixes the slug typo in parts 3 and 4 of the Godot tutorial (/tutorials/Godot/). The capitalised URLs can't be redirected, because on a case-insensitive filesystem the redirect and the page are the same file. If anyone adds a redirect that differs only by case, redirects.ts fails the build with an explanation.

2. Point in-repo links at real pages

https://spacetimedb.com/templates/astro-ts renders the template's README, and its "Chat App Tutorial" link goes to /docs/intro/tutorials/chat-app, which 404s. The same broken links are in 19 template READMEs.

The cause is tools/templates/generate-template-readmes.ts, which had two bugs:

• It built URLs from file paths and ignored slug.
• (\.\.\/)* keeps only its last repeat, so ../../ resolved as ../. That's where the stray /intro/ came from.

It never opened the target file, so neither bug raised an error. Both are fixed, and a link to a missing doc now fails the run.

Hand-written READMEs, Rust doc comments and C# codegen output also used pre-migration paths (/sdks/c-sharp, /modules/rust/quickstart, /sql, /docs/#client). Docusaurus's broken-link check can't see a URL written out in full outside the docs site. These links now point straight at the real page instead of relying on redirects.

Notes for review:

• Template READMEs: only the links changed. Rerunning the generator would also pull in content drift from the quickstarts and leak MDX (<Tabs>, :::warning) into basic-cs. That's a separate problem, left alone here.

• C# SQL link: fixed in crates/codegen/src/csharp.rs, its snapshot, and every checked-in SpacetimeDBClient.g.cs, so the generated files still match codegen.

• Judgement calls:

  • /docs/#client and #host → /intro/key-architecture#client and #host.
  • The C# "module library reference" (/modules/c-sharp) → /core-concepts, since there's no per-language page any more.

• Extra redirects: 7 more, for URLs this repo published but the site never served:

  • /intro/core-concepts/clients/*-reference
  • /reference/cli-reference
  • /reference/sql-reference
  • /sdks/csharp/quickstart
  • /install → https://spacetimedb.com/install

  Projects created from a template keep their own copy of the README, so those links will stick around.

API and ABI breaking changes

None.

Rollback safety impact

n/a

Expected complexity level and risk

2. This is mostly links and docs config. The risk is in the docs build:

• Folder-path redirects are generated at build time, so moving a page updates them automatically.
• The legacy redirects are hand-mapped. The plugin fails the build if a redirect target isn't a real page, so moving a page one of them targets will break the docs build until the entry is updated. That's the intended safety net.

Testing

These checks were run before rebasing onto current master.

• The docs build passes with the plugin enabled, which proves every internal redirect target exists. I checked the generated stubs, including the external /install target.
• I checked all 116 distinct spacetimedb.com/docs URLs in the repo against the build output. Every one is a real page (not a redirect stub), and every #anchor exists.
• cargo test -p spacetimedb-codegen --test codegen test_codegen_csharp passes. submodule_reducer_wire_name_is_qualified_once failed locally because the TypeScript SDK's npm dependencies weren't installed. It's unrelated to this change.
• Prettier is clean on the changed TypeScript.
• After deploy, confirm a legacy URL (e.g. /docs/unity/part-1) and a folder-shaped URL (e.g. /docs/intro/tutorials/chat-app) redirect on the live S3 endpoint.
• Confirm the templates page picks up the fixed links (e.g. "Chat App Tutorial" on /templates/astro-ts).