docs: fix broken README model links breaking docs build

[ko3n1g]

Background

The release / finalize / build-docs job fails the Sphinx build with --fail-on-warning:

README.md:220: WARNING: 'myst' cross-reference target not found: 'src/megatron/bridge/diffusion/models' [myst.xref_missing]
README.md:221: WARNING: 'myst' cross-reference target not found: 'src/megatron/bridge/models/ernie' [myst.xref_missing]
README.md:221: WARNING: 'myst' cross-reference target not found: 'src/megatron/bridge/models/ernie_vl' [myst.xref_missing]
build finished with problems, 3 warnings (with warnings treated as errors).

docs/index.md pulls the root README in via {include} ../README.md, so MyST resolves the README's relative links as cross-references. The three Diffusion/Ernie rows in the Supported Models table linked to bare source directories, which have no doc target and fail resolution.

What changed

• Convert the Diffusion / Ernie / Ernie-VL source-directory links to full GitHub URLs.

Details

• The repo already uses full https://github.com/NVIDIA-NeMo/Megatron-Bridge/tree/main/... URLs for every other source-code reference in the README (e.g. model bridges, PEFT, recipes); rows backed by a doc page keep their docs/models/<family>/index.md link. This change just makes the three offending rows follow the same convention.
• External URLs are not treated as MyST cross-references, so the --fail-on-warning build passes.
• Verified no other bare relative source/example/script links remain in the README.

[claude]

LGTM — the three relative source-directory links are correctly converted to full GitHub URLs, matching the convention already used for every other source-code reference in the README (e.g. line 213). Target directories all exist on main.

Suggested test cases

No perf tests impacted. This is a docs-only change. The relevant validation is the Sphinx --fail-on-warning build in the release / finalize / build-docs CI job.