Document regex for validator party hint

[canton-network-da]

[ This issue was auto-migrated from DA's internal repo (DACH-NY/canton-network-node#17809). Original author: @moritzkiefer-da ]

Somewhere in the validator docs

[a-zA-Z0-9_]+-[a-zA-Z0-9_]+-[0-9]

[canton-network-da]

[ from @waynecollier-da ]: Wherever we add this in the docs, let's also reference it on the Overview page as well.

[canton-network-da]

[ from @martinflorian-da ]: And lets clean up this comment thread once we've applied to the GSF-hosted docs: global-synchronizer-foundation/docs#8 (comment)