Update join guidelines

At the Alpine City Sprint we (@ericof @fredvd @mauritsvanrees and me 😄 ) discussed that we need to update the guidelines on who and how to become a member of collective organization.

The action items where:

- update the guidelines in [collective.github.io](https://github.com/collective/collective.github.io), see [index.html](https://github.com/collective/collective.github.io/blob/master/index.html)
- create a news item on https://plone.org
- create [an issue template](https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/configuring-issue-templates-for-your-repository) in [Collective](https://github.com/collective/Collective) repository

Did I miss anything? 🤔

Start with index.html in the linked collective.github.io repository to review the current membership guidelines. Then inspect the Plone news-item process and the Collective repository's issue-template configuration. Done means the guidelines are updated, a plone.org news item is created, and the issue template is added.

Written by the indexing model from the issue text.