doc: Document on how to start specific daemons with unified config in topotests

[ton31337]

E.g.:

for _, (rname, router) in enumerate(router_list.items(), 1):
    router.load_frr_config(os.path.join(CWD, "{}/frr.conf".format(rname)))

[ton31337]

@Mergifyio rebase

[mergify]

rebase

❌ Pull request can't be updated with latest base branch changes

Mergify needs the author permission to update the base branch of the pull request.
@opensourcerouting needs to authorize modification on its head branch.

[donaldsharp]

I saw some new lldp test failures when I looked. Something is awry

[ton31337]

Where do we use lldp?