Follow-up cleanup to the unified-default flip (#8802, #8803, #8804).
Unified rendering is now the default and the unified property is
@deprecated/@ignore, so the public API docs and examples shouldn't be
framed around "unified mode" any more — it's just gsplat rendering.

Engine source:
- GSplatComponent: remove the "## Unified Rendering" section from the
  class JSDoc; strip the JSDoc blocks from material get/set and
  highQualitySH get/set (both are legacy-only and only documented the
  unified/non-unified distinction). Drop "unified" qualifiers from
  workBufferUpdate, workBufferModifier, setParameter, and
  getInstanceTexture JSDocs.
- GSplatComponentSystem: drop "in unified mode" from the
  material:created and frame:ready event JSDocs, and from the
  getMaterial JSDoc.
- GSplatParams: rename "Parameters for GSplat unified system" to
  "Parameters for the GSplat system"; drop "unified" qualifier from
  dataFormat, material, and format JSDocs.
- GSplatContainer: drop "non-unified rendering" from the class example
  comment, drop unified: true from the addComponent example, drop
  "unified" qualifier from the centers JSDoc.

Examples (gaussian-splatting folder):
- Remove unified: true from every addComponent('gsplat', { ... }) call
  (~40 occurrences across 29 files) — it's now the default and the
  property is deprecated.
- Reword/remove comments that mention "unified" framing now that it's
  just the standard path (e.g. "Create hotel gsplat with unified set
  to true" → "Create hotel gsplat").
- Drop three stale orbit-pivot comments ("unified gsplats have no mesh
  AABB for focusEntity framing") — these became factually incorrect
  once #8803 made orbit-camera unified-aware.
- Update global-sorting example header from "demonstrates unified
  gsplat rendering" to "demonstrates global gsplat sorting" to reflect
  what the example actually shows.

Remaining "unified" mentions in the engine are all either internal
(private fields, code comments, code branches), runtime warning strings
(which must keep the property name), @ignore'd JSDoc, or directory
names — none render in public docs.

Co-authored-by: Martin Valigursky <mvaligursky@snapchat.com>

Restructures the Gaussian Splatting documentation now that unified splat
rendering is the default (and only) mode, following engine PR
playcanvas/engine#8805 which deprecates the `unified` GSplatComponent
property. Framing the docs around "unified vs non-unified mode" no longer
makes sense. This is a structural first pass; individual page bodies are
cleaned up in follow-ups.

- Moved the former building/unified-rendering/ topics up to the building/
  level and removed the now-empty folder.
- Merged the standalone "Unified Splat Rendering" page into "Splat
  Rendering Architecture", rephrased to present splat rendering as the
  single rendering mode; deleted the old page.
- Added a new top-level Rendering Architecture section holding the
  pipeline-internals topics (overview, Splat Data Format, Work Buffer
  Format, Work Buffer Rendering, Splat Processing).
- Reordered Building Splat Applications into a more logical flow.
- Repointed all internal links and added redirects from original
  published URLs to their new locations.
- Mirrored every change in the Japanese (i18n/ja) docs.

Co-authored-by: Martin Valigursky <mvaligursky@snapchat.com>