Add documentation for velocity support in coordinates - #6260

• Note that I'll also be updating this with docs for Support Velocities in coordinates by having transforms that use finite differences #6226

• Also, this will get the docs for Add bary/heliocentric radial velocity correction #5752 since that's (indirectly) related work and fits logically in the same doc sections.

Just had a look, and like the initial update. I think we'd need some examples, though, which ideally show some obviously correct transformations (like a PM in pure galactic longitude near the galactic centre, so one sees where it is heading in RA, Dec).

Also, as discussed in #6219, clean up mentions of "must be keyword" in Frame class docstrings.

• Synergize with placeholder "what's new" section on the velocities

As this is supposed to be docs only I'll go ahead and cut the RC before it. But I'll wait a few more hours in the hope that either @adrn or @eteq wakes up and can rebase this.

Right, don't merge yet - we still have some more docs to add!

OK, this is useful info as well, so I'll go ahead with the RC and will merge this when ready for the actual release.

• the galactocentric.rst page still needs updating to include information about velocities.
• we should add an example showing how to set the Galactocentric v_sun with the proper motion of Sgr A* rather than a 3D cartesian velocity

@adrn - OK, populated "my" sections from #6260 (comment) and those right after, so I think it's just the two you noted above. The "what's new" could maybe use some beefing up, but I'm unsure what to put beyond what's there... open to ideas, but I'm also willing to accept that as it stands here.

Oh, but one other thing to add either here or in #6317 (whichever gets merged second):

• Example of the "easy" differential-choosing from Allow specifying differential class in frame init #6317 (e.g. ICRS(..., pm_ra=...,differential_cls=SphericalDifferential)

Note: this now contains commits from #6329

I added two examples to this PR, but it really could use one more: an example of how to subclass BaseCoordinateFrame to implement a position and velocity offset (e.g., shift to the Milky Way-M31 barycenter). Not sure I'll have time to implement this before tomorrow, though...

Hi there @adrn 👋 - thanks for the pull request! I'm just a friendly 🤖 that checks for issues related to the changelog and making sure that this pull request is milestoned and labelled correctly. This is mainly intended for the maintainers, so if you are not a maintainer you can ignore this, and a maintainer will let you know if any action is required on your part 😃.

Everything looks good from my point of view! 👍

Once the tests pass, we should go ahead and merge this so that we have this in (if nothing else, for the what's new), but we can definitely still try to squeeze in another example in a separate PR if there's time before the release is finalized, @adrn