docs(assets): add Custom Asset Parsers page

[mvaligursky]

Important

Merge when engine v2.21 is released — the documented API (ResourceHandler#addParser and the parser registry) ships in that release.

Documents the pluggable parser registry added to the engine's resource handlers (playcanvas/engine#9001, with follow-ups #9004, #9005, #9006), which was previously undocumented territory: there was no page covering ResourceLoader / ResourceHandler / parsers at all.

Changes:

• New page User Manual → Assets → Custom Asset Parsers (EN + JA):
  • how asset loading works (loader → handler → newest-first parser selection)
  • the parser contract: canParse(context) with a ParserContext reference table, load via the this.handler.fetch helper, optional open
  • registering parsers, override semantics (later registrations win over built-ins), removeParser / parsers
  • a complete TGA texture parser example, including the texture family's extended open(url, data, device, textureOptions) signature and the spread-last options convention
  • per-handler notes: the 16 participating handler types, material/audio specifics, and which handlers ignore parsers (orchestration/special)
  • pointer to the engine's OBJ parser sample and loader example
• supported-formats.md (EN + JA): new "Custom Formats" section linking to the page
• sidebars.js: page registered after Supported Formats