Add a Transparency page to the graphics user manual

[mvaligursky]

The graphics user manual has no conceptual coverage of transparency. Alpha to coverage in
particular appeared only as a single row in the editor inspector and web components reference
tables, and blending, alpha test and opacity dithering were not explained anywhere. This adds a
page that covers all four and, more importantly, when to choose each.

Changes:

• New Transparency page under user-manual/graphics, placed after Physically Based Rendering
  in the sidebar, covering alpha blending, alpha test, opacity dithering and alpha to coverage.
• Each technique notes whether it is order dependent, which pass it renders in, and its main
  drawback, with a summary table and a short "which one should I use" guide.
• Documents that alpha to coverage requires a multi-sampled render target and is silently
  ignored otherwise, which is the most likely thing to be reported as a bug.
• Documents the WebGPU-only requirement that the first color attachment use a blendable format
  with an alpha channel, and that CameraFrame defaults to PIXELFORMAT_111110F, which has
  none - so alpha to coverage does nothing through CameraFrame unless a format with alpha is
  requested. Also notes that WebGL has no such restriction, so this is a deliberate difference
  between the backends rather than a bug.
• Japanese translation added alongside, keeping the graphics section at full ja parity.

Note on timing: the alpha to coverage section describes behaviour that requires engine
2.22 (playcanvas/engine#9208, which wires the flag into the WebGPU pipeline). The rest of the
page documents existing behaviour. Please hold until 2.22 ships.

Follow-up needed: two cross-links with #1097 (dual-source blending)
are deliberately omitted, because the dual-source page does not exist on main yet and
onBrokenLinks defaults to throw, so linking it now would fail this PR's build. Once #1097
merges, the blend state section should link to the dual-source page, and the dual-source page
should link back here. Whichever of the two merges second is the natural place to add them.

[mvaligursky]

Superseded by #1097. Rebased that branch onto main and moved the Transparency page into it, so the two pages can cross-link - while they were separate branches neither could link to the other's page without failing the build (onBrokenLinks defaults to throw). Branch mv-transparency-docs left in place; nothing lost.