Add stagit lister

It's a cgit/gitweb like technology

"stagit is a static page generator for git." [1]

Instances example [2]

Plan:

• swh/devel/swh-lister!486 (merged): Implementation
• swh/devel/swh-environment!273 (merged): List various instances in docker (see mr description)
• Deploy in staging
• Deploy in production
• (ongoing) Ingest forges [2]
• swh/devel/swh-web!1161 (merged): Reference in archive coverage widget and add type to add-forge-now ui

[1] https://codemadness.org/stagit.html

[2] (Thx @guillaume for the listing again ;)

• https://git.acid.vegas/
• https://a3nm.net/git/
• https://www.brianlane.com/git/
• https://git.codemadness.org/
• https://git.tronto.net/
• https://git.2f30.org/