Add changelog for 3.0.0

[consideRatio]

I'd like to hold of a bit with the release for these issues/prs that can be resolved relatively easy I think. I'm trying to nudge them towards completion for a day or two and then go unblock again.

• Support {server_base_url} variable substitution #565
• [KubeIngressProxy] Add a long docstring to KubeIngressProxy class #568
• Misc maintenance details #571
• [KubeIngressProxy] Add underscore prefix to private functions: safe_name_for_routespec and delete_if_exists #572
• Replace recommonmark with myst_parser and fix changelog rendering #575
• kubernetes_asyncio regression: AttributeError("'ClientResponse' object has no attribute 'getheader'") in ResourceReflector #580 that led to reporting an upstream regression: Regression via recent code regeneration tomplus/kubernetes_asyncio#183
• Failing tests against JupyterHub 2.2.0 #582 (important)

[manics]

Is the readthedocs workflow working?

https://jupyterhub-kubespawner--581.org.readthedocs.build/en/581/index.html
is missing several several entries in the left-hand contents bar, compare

with
https://jupyterhub-kubespawner.readthedocs.io/en/latest/

On the other hand https://jupyterhub-kubespawner.readthedocs.io/en/latest/index.html is missing the 2.0 release!

[manics]

On second thoughts I think the RTD discrepancy is because #575 isn't included in this branch, and RTD presumably builds off the branch not the PR merge commit. Any chance you could rebase this so we can see the final changelog properly?

[consideRatio]

I now consider KubeSpawner 3 ready for release!

[manics]

A couple of suggestions, otherwise looks fine to release!