docs: move design/roadmap docs to the wiki, trim the repo root

The repository root carried 11 large Markdown files (~330 KB); most were
long-lived design records rather than things a newcomer needs on first
contact, which buried the README and mixed "how to use Ludic" with "how we
decided to build it."

Move the design/roadmap docs to the Forgejo wiki (now enabled and
populated): Events, Networking, Scenes, Lifecycle, Mobile and
Syntax-redesign design records, the Bootstrap deep-dive and the Luanti
roadmap, under a Home index + sidebar. Each page had its selfhost/ source
links corrected for the #29 reorg and every repo-relative link rewritten to
an absolute URL on main so it resolves from the wiki.

All eight were current, actively-maintained records, so none were dropped.
The root now holds README.md plus the two user-facing references,
LANGUAGE.md and COMPILING.md; the README links to the wiki, and the
remaining references in LANGUAGE.md / COMPILING.md / examples/README.md and
the emit_net.ludic header comment point at the wiki pages. The emit_net.ludic
change is a comment only — the seed stays byte-identical and bootstrap-cfree
+ the full suite (56) stay green.

Closes #26

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

selfhost/backend/stdlib/emit_net.ludic

@@ -1,4 +1,4 @@
-# emit_net.ludic — NETWORKING N2–N6 codegen (NETWORKING-DESIGN.md).
+# emit_net.ludic — NETWORKING N2–N6 codegen (see the Networking design on the wiki: Design/Networking).
 #
 # A program that calls net_send/net_poll with no `extern fn` override triggers the
 # built-in loopback transport; this flag defers its emission to end-of-module.