[feature] Show Description and Terms of an instance on the landing page

[RiverSongFox]

Is your feature request related to a problem ?

Full description of an instance can be edited at settings/admin/instance-settings but is not displayed anywhere. It might be beneficial to display it under the short description on the landing page.

Describe the solution you'd like.

Extend web/templates/index.tmpl with instance.Description.

Describe alternatives you've considered.

One option is to edit web/templates/index.tmpl template manually for a particular instance.

Cramping everything into short description is also possible because it accepts HTML: https://github.com/superseriousbusiness/gotosocial/blob/main/internal/processing/instance.go#L184-L191

Additional context.

• The landing page itself was created in #32
• The instance.Terms Terms & Conditions is not printed as well, it may be convenient to add it too

[tsmethurst]

Closing this as a duplicate of https://github.com/superseriousbusiness/gotosocial/issues/143