docs: Set up Doxygen + Doxygen Awesome for SDK reference documentation

[kirre-bylund]

Summary

Sets up Doxygen with the Doxygen Awesome CSS theme as the documentation generator for the Unity SDK, and adds a CI workflow that packages and publishes the generated docs.

Closes lootlocker/index#1461
Part of lootlocker/index#1203

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Changes

.doxygen/ — all generation config (export-ignored, not shipped with SDK)

• Doxyfile — input: Runtime/, excludes Libraries/Editor/internal files, output: docs/html/; EXTRACT_ALL=NO / HIDE_UNDOC_MEMBERS=YES; INTERNAL_DOCS=NO; JAVADOC_AUTOBRIEF=YES
• doxygen-awesome/ — doxygen-awesome.css + doxygen-awesome-sidebar-only.css from jothepro/doxygen-awesome-css
• custom-layout-fixes.css — 500px sidebar, branding padding, sidebar background extension
• lootlocker-theme.css — LootLocker colour overrides
• nav-customization.js — two-phase nav tree restructuring: enums into Types, Topics hoisted to root, namespace prefixes stripped from sidebar labels
• DoxygenLayout.xml — Types before Topics; pages/modules/namespaces hidden
• header.html / footer.html — custom HTML with script injection for nav-customization.js
• images/lootlocker-logo.png — branding

.github/workflows/generate-docs.yml

Trigger	Behaviour
PR to main or dev	Runs Doxygen — validates no errors
Push to main	Generates docs and uploads sdk-docs-{sha} artifact (30-day retention)
Release published	Generates, zips, and attaches docs-{version}.zip to the GitHub release
Manual dispatch	Generates docs on demand

.gitattributes

• .doxygen/ added to export-ignore — generation config is stripped from archives and package exports; only docs/html/ (generated output) ships with the SDK

.gitignore

• docs/html/ excluded from version control (generated output)

package.json

• documentationUrl points to hosted docs — Unity Package Manager surfaces this as the "Documentation" button

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Verification

Ran locally with Doxygen 1.16.1:

doxygen .doxygen/Doxyfile

• Exit code: 0
• Warnings: 0
• Errors: 0
• Internal types (Libraries, Editor, EndPoints, Obfuscator, Validation) confirmed absent from output