docs: clarify when to pull Quay.io image vs build locally

[CoderLink06]

This PR adds a short clarification to help new users decide when to pull an existing image from Quay.io versus when to build a custom image locally.

It is based on the discussion in issue #2395 and aims to make the image selection workflow clearer for first-time users.

Changes:

• Added a brief "Pull vs build" guidance block to docs/using/selecting.md

[CoderLink06]

I added a small follow-up clarification to docs/using/running.md explaining host vs container port mapping for the -p 10000:8888 example, since this seems to be a common source of confusion for new users.

Happy to adjust or remove if you’d prefer this to live elsewhere in the docs.

[mathbunnyru]

I added a small follow-up clarification to docs/using/running.md explaining host vs container port mapping for the -p 10000:8888 example, since this seems to be a common source of confusion for new users.

Happy to adjust or remove if you’d prefer this to live elsewhere in the docs.

I think it's already explained quite well, and I haven't seen any questions about it.
Let's keep docs like this for now, because providing too much information is also not the best thing.