This one could be interesting in the context of the multiple wikis
scenario, where they could all aggregate in a single site. But alas,
it only supports asciidoc.

Closes: #90.

but at least we're in sync with the template now

at least one user was thrown off guard and thought the docs here were
irrelevant because of this section

i actually redid the mirror procedure with this setting enabled to see
if this commit will overwrite a test commit I just did on the wiki,
let's see...

and GIT_SUBMODULE_STRATEGY as the suggested values are already used as
the default values, document instead in Design section (next commit)

Those can still be useful as a disaster recovery mechanism.

This reverts commit deb01b72.

This script was used only in the previous webhook implementation but it
was superseded and is not deployed anymore so these instructions cannot
work currently.

This mainly adds the important bits for working with the new review apps
and staging environments in GitLab CI.

Details at tpo/web/team#15