Document the MediaWiki:Maps config page and preload an example

[JeroenDeDauw]

Adds documentation affordances to the MediaWiki:Maps on-wiki configuration page.

Editing the page suppresses the default MediaWiki-namespace intro and frames the JSON editor with a one-line pointer to the on-page reference and the external documentation, and the generated reference below the editor.

Viewing the page trims the rendered output to the JSON table and adds the same pointer and reference.

Creating the page preloads a small valid example: two general settings, a coordinate setting and one custom Leaflet layer. A test pins that it passes the config validator, so a schema change cannot leave the preload invalid.

The reference is generated from ConfigSchema, never hand-written, so it cannot drift from the settings actually exposed. Per group it renders a table of the page key, the accepted value (described by the value type itself) and the LocalSettings.php setting it overrides; the reference links the documentation once, from the pointer above it, so no per-setting prose is duplicated on the page.

All three affordances apply only to the MediaWiki:Maps title and only when $egMapsEnableInWikiConfig is true; with the kill-switch off the page is a plain wikitext page, consistent with the existing config hooks.

Considered, omitted:

• A dedicated ResourceLoader styles module: the reference reuses the core wikitable class.
• Per-setting prose in the reference: semantics live in the external documentation, found via the setting name in each row.
• Translated group headings: the headings are the literal JSON keys an administrator types, shown verbatim.
• An intro paragraph in the reference: the pointer above it already links the documentation.
• A default-value column: deferred; defaults for the nested types are verbose, and the preloaded example already shows concrete starting values.

_{AI-authored — Claude Code, Opus 4.8 (max); detailed brief from a Fable session (@JeroenDeDauw), no human steering mid-task; diff not yet human-reviewed; unit + integration tests written and run locally (green) and mutation-verified, edit and view rendering verified in the browser on the dev wiki, CI green.}