docs(wiki): migrate documentation out of repo into GitHub Wiki

The 84 markdown files under docs/wiki/ are now published to the
project's GitHub Wiki at https://github.com/kingchenc/wickra/wiki —
a separate git repository (https://github.com/kingchenc/wickra.wiki.git)
that GitHub hosts natively with its own UI, search and history. The
flat layout that the GitHub Wiki requires has been generated, all
internal cross-links rewritten, and a _Sidebar.md groups the 71
indicators by their canonical 8 families.

Effects:
- docs/wiki/ is removed from the main repo (-84 files). docs/README.md
  now just points readers at the Wiki.
- PR template + CONTRIBUTING text updated to point at the Wiki instead
  of the in-repo path. The Wiki repo is separately cloneable and
  editable via the GitHub web UI.
- examples/wasm/README.md cross-link fixed to use the Wiki URL.
- The (still in-repo) CHANGELOG keeps its historical references to
  docs/wiki/ paths — those describe what the tree looked like at past
  releases and stay accurate as history.
- README.md, license, all source unaffected.

The Wiki itself ships with _Sidebar.md / _Footer.md generated from the
8-families taxonomy and 503/503 cross-links resolved.

.github/PULL_REQUEST_TEMPLATE.md

@@ -24,8 +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).
-- [ ] Documentation under `docs/wiki/` and the `README.md` is updated
-      (if applicable).
+- [ ] The relevant page on the [project Wiki](https://github.com/kingchenc/wickra/wiki)
+      and the `README.md` are updated (if applicable). Wiki edits go to a
+      separate repository: `https://github.com/kingchenc/wickra.wiki.git`.
 - [ ] An entry was added under `## [Unreleased]` in `CHANGELOG.md`.
 
 ## Notes for reviewers