dev: build HTML from src/oas.md

[ralfhandl]

• new npm script npm run build-src to build HTML from file file src/oas.md
• produce deploy-preview/oas.html
• git-ignore folder deploy-preview
• updateCONTRIBUTING.md from main: dev: update from main #4271
• describe build scripts in CONTRIBUTING.md

Part of

• Create dev build infrastructure #4223

[handrews]

I tried this out and it works as advertised- thanks!

One question: Could we put src-preview/ under deploy/? Or maybe have a deploy-dev/? I kind of like keeping the build artifacts together, or at least following the same sort of naming. But this isn't something I'd insist on, just a preference.

[ralfhandl]

We can put oas.html wherever you like, I just thought it is easier to find next to src/, and we could also build the JSON Schema files from src/schemas/validation/ into it.

[lornajane]

We agreed to name the output directory deploy-preview (the main build script outputs to deploy/. Also this pull request needs some usage documentation in the CONTRIBUTING file so we can remember what we built and how to use it.

[handrews]

it seems odd to me that we're putting a tempfile under /deploy when our output is under /deploy-preview. Should we not have the tempfile there and named something obvious (eg. with tmp or temp in the name?) so that if something breaks the mess is left in the same place we were expecting output? And we can have an npm run clean-src that just deletes that directory so that it works whether the build was successful or not? (I don't care if you add clean-src in this PR or not, I'm just thinking it might be nice at some point).