Add description of UKPA membership to uk.python.org

[rebkwok]

2023 ticket sales were referencing an old PyCon UK page for details about the UKPA: https://2019.pyconuk.org/ukpa/

That was updated to point to uk.python.org instead, which contains much of the same information. The one piece of info that may be missing is a note of how membership works:

Anyone attending the conference is entitled to membership of the UKPA and is thus entitled to vote at the AGM. Membership will expire after 12 months or at the 2020 conference - whichever is sooner.

Consider adding a similar (generic) note on uk.python.org (in line with constitution wording)