browse: Add help page and some ui improvements

swh/web/browse/urls.py

swh/web/browse/views/content.py

swh/web/templates/api-endpoints.html

swh/web/templates/api.html

swh/web/templates/browse.html

swh/web/templates/homepage.html

swh/web/templates/includes/apidoc-header.html

swh/web/templates/includes/apidoc-header.md

swh/web/templates/includes/browse-help.html

swh/web/templates/includes/origins-search.html

swh/web/templates/revision-log.html

swh/web/templates/revision.html