Implement syncing Vale definitions between this repo and mautic/mautic-documentation

[alanhartless]

Vale has been implemented in both repositories and thus have the same definition files that must be kept up to date. It would be better to keep these in sync. Maybe we can store the definitions in another repository and use git submodules (each repo would still need to pull the latest hash) or use some Github action magic.

[RCheesley]

Maybe we can use the https://github.com/mautic/.github repo for this purpose?