sysadm: Move network documentation to sysadm instance
This also:
- installs redirection from old location to the new one.
- installs intended audience on impacted docs
- fixes typos
- fixes broken links
- wraps text as usual (88 chars)
Related to swh/infra/sysadm-environment#3154 (closed)
Test Plan
make -C docs html
make -C sysadm html
still happy.
Migrated from D6508 (view on Phabricator)