Add process and pull request template for nominating voting members

[louwers]

Based on https://github.com/wipfli/maplibre/pull/1

• This adds a pull request template for creating a nomination for a voting members, so people can select this template when making a pull request, so they don't need to copy paste the template by hand.
• Explanation of the process written by @wipfli is added to VOTING_MEMBERS.md.
• I created a new label voting-member-nomination which is added by the template link so people have an overview of all nominations and can vote on the ones they want.

Draft pull request, the HubSpot link needs to be added in two places.

[louwers]

Let's try it out once we merge it here.