Mention/add npm run vendor in our frontend dev steps

[stsewd]

We don't update our vendored packages that often, but when needed, I had no idea how to update them, we lost that step when migrating the process to docker.

See

• Embedded JS: Conditionally inject jQuery #9861 (comment)
• https://github.com/readthedocs/readthedocs.org/blob/3.11.0/docs/development/front-end.rst

[humitos]

This probably won't required anymore since we are migrating all the frontend things to addons and ex-theme. I don't we will need to rebuild the frontend things from this repository anymore.

Also, I think the current pattern is just running inv docker.buildassets

[stsewd]

Also, I think the current pattern is just running inv docker.buildassets

That doesn't run npm run vendor

https://github.com/readthedocs/common/blob/e6da5905c17254b5e8d78adc20e77a88f51f41f1/dockerfiles/tasks.py#L165-L169

I haven't been following the frontend changes, so up to you if you thinks this is still needed.