Docs for updating docker images

[sjspielman]

Closes #497

This PR adds docs that describe why you need to file 2 PRs to update a docker image, once all the GHAs have been turned on. Note that it contains one #STUB_LINK which can be completed once #533 goes in.

Instructions for reviewers:

1. I am not at all sure if I have presented this information clearly, so please let me know whether there is a different logical flow of information which would make this easier to understand! Any comments about clarity and presentation are appreciated!
2. Are there any additional details to communicate besides that they need 2 PRs?
3. Are there any more spots in the docs that would benefit from linking to this page?

[sjspielman]

I've rearranged the page in a way I think makes things clearer, and bonus removes the need to distinguish implement vs activate.

• I included an admonition at the top explaining that these instructions only apply when workflows are activated. This way, I never have to say anything about that caveat again on the page!
• I changed some of the framing/language added more subsections, including one that needs header help! "Context" seemed too thin, but I couldn't think of anything else?

[sjspielman]

Noting this link check fail was expected https://github.com/AlexsLemonade/OpenScPCA-analysis/pull/547/commits/6dce7a46d738c47cc8457dcf6a7c14395dec9651. It has STUB_LINKs for docs incoming in #533.

[sjspielman]

(this was me checking my creds rotation worked :) https://github.com/AlexsLemonade/OpenScPCA-analysis/pull/547/commits/6d28768ac380584b0c0161096b01b7db4128fd94)

[sjspielman]

@allyhawkins is this where you had in mind for the note? https://github.com/AlexsLemonade/OpenScPCA-analysis/pull/547/commits/f22c07174fdcc53a93577f1966152ebe3648264a

[sjspielman]

Again noting that link failures are expected, so we don't need that to pass before merging.