docs(config): add configuration reference tables and fix links_space_char default

Document every config key (top-level, mappings, per-wiki, HTML export, and
g:nuwiki_* globals) with types, defaults, and accepted values in the README
Configuration section. Correct the links_space_char doc comment: the default
is a literal space (spaces kept verbatim), not "_". Note that
diagnostic.link_severity is accepted but not yet wired through to the server.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

crates/nuwiki-lsp/src/config.rs

@@ -65,7 +65,8 @@ pub struct WikiConfig {
     /// that many extra columns. Layout hint for the renderer.
     pub list_margin: i32,
     /// Character used in place of literal spaces inside wikilink
-    /// targets when writing to disk (`_` by default).
+    /// targets when writing to disk (a single space `" "` by default,
+    /// i.e. spaces are kept verbatim).
     pub links_space_char: String,
     /// `nested_syntaxes` — code-block language → editor filetype map
     /// used for syntax-highlighting fenced blocks. Empty by default.