[FEAT] Add GitHub action to sort/update custom dictionary

[zslade]

Is your proposal related to a problem?

custom_dictionary.txt is updated as part of the manual spellchecking process but it's time consuming to add words in alphabetical order.

Related to PR: #2025

Describe the solution you'd like

Set up a GitHub Action to automatically alpha-sort the custom dictionary when updates to it are merged into master and/or splink4_dev.
Removing duplicates from the dictionary could also be done as part of this Action.

This information also needs to be reflected in the documentation on using the spellchecker.

Describe alternatives you've considered

Additional context

[RobinL]

I think this is possibly overkill in terms of infra complexity given that an unsorted file doesn't actually break anything

When I added some terms I used the sort command in vs code:

[zslade]

Makes sense, thank you. Happy for this issue to be closed and I can just update the docs accordingly 👍

[RobinL]

thank you! (sorry was just doing a bit of a more general issue review this morning and spotted this one )