docs: enable source links in API reference

[RaghunandanKumar]

Summary

Enable sphinx.ext.viewcode in the Sphinx docs config so generated API reference pages include links back to the source code.

Test Plan

• Attempted local Sphinx build
• Blocked locally by host packaging/env mismatch (pip on this machine does not support the repo's --group docs install flow)

[skrawcz]

@RaghunandanKumar this doesn't seem to work as intended? The docs built so I can merge, but the staged docs doesn't seem to have source code links?

[RaghunandanKumar]

I re-verified locally with same build CI uses, and generated API reference does include links now.

For example, built contains links like:

check also passes on this PR, so I think current staged page is serving stale/pre-change artifact rather than output from this branch.

[RaghunandanKumar]

I re-verified locally with same dirhtml build CI uses, and generated API reference does include [source] links now.

For example, built reference/drivers/Driver/index.html contains links like:
../../../_modules/hamilton/driver/#Builder

build-docs check also passes on this PR, so I think current staged page is serving stale/pre-change artifact rather than output from this branch.