Convert absolute external links to cross-project references #7

[Maalvi14]

Made external links to cross project references as mentioned in #7.

Summary:

What changed

• Replaced many C++ API links (class/method pages) with {usdcpp}\... so they resolve through USD.tag instead of pasted URLs.
• Switched several OpenUSD docs / tools / glossary-style targets to inv:usd:std... (e.g. FAQ doc, toolset entries like usdcat, user_guides where it worked).
• Touched ~19 lesson/glossary-related files under docs/ (stage-setting, composition, creating-composition-arcs, beyond-basics, scene-description-blueprints, data-exchange, glossary).

What I did not fully convert

• Kept some absolute URLs where inv: / {usdcpp} had no stable inventory or tag entry (e.g. certain API “page front” / annotated-style pages), so the build would stay warning-free.
• Left intentional openusd.org in docs/conf.py (intersphinx base URL) and docs/index.md (homepage link).

Verification

Ran uv run sphinx-build -M html docs/ docs/_build/ so cross-refs are exercised at build time; final state was clean on that run after settling ambiguous targets.

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Commit signed off as usual.

[JenNVIDIA]

Sorry for the delay! Merging, Thank you for your contribution!