Add a page about procedure to follow to patch old releases

let's say you need to patch pipeline 1.0 because that's the one in prod, so you want to patch it.
I'd create a 1.0 branch based on the 1.0 release, make a PR which fix the bug, if said bug still exists in dev, make a PR to dev to fix it too.
If necessary pin nf-core tools to ensure linting works.
Merge PRs and release from the 1.0 branch with a 1.0.fix.0 tag.
If another patch need to happen on this patched release, then continue working on said 1.0 branch, and do a 1.0.fix.1 tag

I think that we should only accept bugfixes for patch releases if the bugfix also goes into dev. I think that it needs to be applied twice, no way around that. (except if the bugfix is already in dev or no longer relevant)