docs(P6.4): repoint all doc links from the GitHub wiki to docs.wickra.org (#86)

The documentation now lives in the wickra-lib/wickra-docs VitePress repo and
will deploy to docs.wickra.org. Rewrite every tracked, user-facing wiki link
in the main repo to the new canonical site:

- docs/README.md, CONTRIBUTING.md, PULL_REQUEST_TEMPLATE.md
- bindings/{node,python,wasm}/README.md, examples/wasm/README.md
- repo-metadata.toml: wiki_url/wiki_git -> docs_url/docs_git

Page paths map 1:1 to VitePress clean URLs (e.g. /wiki/Quickstart-Rust.md ->
docs.wickra.org/Quickstart-Rust). The sync-about.yml wiki-sync step is left
untouched on purpose: it stays until the docs site is live (tracked as P8.3).
The GitHub wiki itself is not deleted yet for the same reason.

.github/PULL_REQUEST_TEMPLATE.md

@@ -24,9 +24,9 @@
 - [ ] New behaviour has tests; bug fixes have a regression test.
 - [ ] Public API changes are mirrored in the Python / Node / WASM bindings
       and their type stubs (If applicable).
-- [ ] The relevant page on the [project Wiki](https://github.com/wickra-lib/wickra/wiki)
-      and the `README.md` are updated (If applicable). Wiki edits go to a
-      separate repository: `https://github.com/wickra-lib/wickra.wiki.git`.
+- [ ] The relevant page on the [documentation site](https://docs.wickra.org)
+      and the `README.md` are updated (If applicable). Docs edits go to a
+      separate repository: `https://github.com/wickra-lib/wickra-docs`.
 - [ ] An entry was added under `## [Unreleased]` in `CHANGELOG.md`.
 
 ## Notes for reviewers