[BEAM-14347] Update docs to prefer generic registration functions

[damccorm]

Now that we've proven out the effectiveness of the generic registration functions in #17613, we should update the docs to recommend that approach. This is the last piece of work for the generic registration to be done for now.

✨ RENDERED ✨

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R: @youngoli for final approval

[damccorm]

I'm actually going to override the bot and send this to Robert since he's been looking at the other registration stuff

R: @lostluck

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[lostluck]

Consider splitting the CHANGES.md messaging out to it's own PR.

Since 2.40 doesn't release until July, the generics registration shouldn't yet be in the Programming Guide, when they can't be used for 6-8 weeks. Website changes are live within a day.

[damccorm]

Oh, forgot that the website doesn't get updated with releases. I opened #17643 and will close here for now

[lostluck]

This reveals we may want to also have a series of register.FuncDoFn for func DoFns, which should be very similar but much much simpler than the struct ones.

[lostluck]

Whoops missed publishing these before you closed it.

WRT documentation changes, we likely do want to hold on making the beam package GoDoc mention these until we're unblocked from the import process.

But the other example code (everything in beam/examples) and integration test code can certainly make use of it before then. Doesn't need to be all at once, we can divide it amongst the team so folks can get used to it.