docs: stop describing an embedded viewer

[NivGreenstein]

Closes MAPCO-11482 (docs half). Server half: MapColonies/shigola#6

Shigola no longer ships a viewer, so the pages telling people to open one are now wrong, not
merely out of date — /viewer returns 404.

What changed

• getting-started.md loses its "Shigola viewer" step. What replaces it is a check that the
  service is actually serving — the landing page and the collections list — plus a pointer to the
  tutorials, which already show how to put the tiles on a map with MapLibre, OpenLayers or Mapbox
  GL. That is now the only way to see them.
• http-endpoints.md drops the GET /viewer/ entry and the -tags noViewer note; the tag no
  longer exists. style.json is no longer described as "used by the embedded viewer".
• ogc-api-tiles.md and about.md stop listing the viewer among the things that surprise
  people, and about.md's credits keep every word of their attribution to Tegola except the viewer,
  which this codebase no longer contains.
• download.md drops the go generate / npm step. Nothing in the build needs node any more.
• support.md's "/ returns JSON instead of the viewer" entry becomes "/ returns JSON, and
  /viewer 404s".
• The 3.3 MB screenshot of the viewer rendering Bonn is deleted.

No migration note

These pages simply stop mentioning a viewer, rather than announcing that one was removed. There is
no stable release to upgrade from, so there is nobody a "breaking change" note would be addressed
to — and documenting the absence of a feature only raises a question the reader did not arrive with.

That means the shigola_viewer_* metric family goes unmentioned too. It deliberately does not
satisfy the ticket's "noted as a breaking dashboard change" wording; see the note on MAPCO-11482.

Verification

npm run build passes — the real check here, since broken links and anchors throw. Run after
npm run clear and a cache wipe, given a file was deleted.

One repair beyond the removal: reordering getting-started.md had left a parenthetical whose
antecedent moved two paragraphs away, and which was never closed (a pre-existing missing bracket).
It now sits with the sentence it belongs to.

Branch is feat/remove-viewer rather than docs/… to pair with the server branch by name; the
commit itself is docs:.

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