Add CI to PRs to build/test

[yarikoptic]

We have a workflow which is triggered only on outages to master since it uses action which pushes to gh pages.

Ideally we need a workflow to just build and ideally to preview in PRs. It seems we use mkdocs. https://github.com/bids-standard/bids-specification too, but with RTD, which provides preview functionality.

So may be we should integrate with RTD?

[yarikoptic]

d'oh -- while sending

• Expand validation docs for dandi-cli#1822 (companion JSONL, grouping, VisiData) #235

I realized we still, in 2026, do not have previews! yikes.