docs: add known-issues.md gathering vimwiki divergences

Consolidate the deferred features and intentional divergences from the
parity log (development/vimwiki-gap.md) into a user-facing reference:
- Deferred: the markdown generated-content cluster
  (markdown heading generation, markdown_header_style, markdown_link_ext)
- By-design divergences grouped by area (highlighting/conceal -> LSP,
  syntax detection, config mechanism, auto_tags, mouse maps, list_margin
  negative + diary_caption_level -1 value semantics, Vim/Win shims)
- Default-location differences (html_path/template_path/diary_rel_path)
- Notes on the vars.vim compat shim and additive commands

Link it from the README "Migrating from vimwiki" section so it's
discoverable.

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

README.md

@@ -664,6 +664,10 @@ modification. The first workspace scan after launch may show an empty
 `:VimwikiCheckLinks` result until the background index completes;
 subsequent runs are instant.
 
+A few upstream options are implemented differently or not at all — by
+design, since nuwiki is LSP-backed rather than pure VimL. See
+[`known-issues.md`](./known-issues.md) for the full list of divergences.
+
 ---
 
 ## License