Add links to "View Extension" page

Add link field for documentation (see how hard it would be to modify module extdir to populate an extension node with the documentation field from the latest release node)
Make the Git URL clickable. This field is currently of type Text, which means we don't have easy options available for making it a link. One easy option is to create a new field of type Link. However, this means we'd have to migrate existing content to this field (probably not awful) and update the extdir module to populate the new field.

These items are grouped together because they both depend on learning more about how extdir works and possibly changing it.

Edited Oct 20, 2020 by bgm