verify website link on mastodon

copied from mastodons web-interface: settings > edit profile > verification:

You can verify yourself as the owner of the links in your profile metadata. For that, the linked website must contain a link back to your Mastodon profile. The link back must have a rel="me" attribute. The text content of the link does not matter.

This change adds a hidden link to F-Droids official mastodon account. Mastodon clients then will display the link to f-droid.org in our mastodon account as verified.

I've also added some boilerplate markup for the new hidden section of the website in case we decide to have a link to our mastodon account present on the websites sidebar.