docs: task check is the check; the private mirrors on the profile

[jshvn]

What changes

• CONTRIBUTING.md and the PR template name task check, the verb that exists, instead of task render.
• SECURITY.md names only the upstreams that exist.
• The profile README lists the two Proton mirrors beside the public ones, describes how an rsync mirror and a Proton mirror run without naming one engine's verbs, and sends a forker to a mirror's "Want your own?" and to lib's README rather than to "your vault, or your repository secret".

Part of the README consolidation; lib's PR is katoptra/lib#5.

Storage and operations

None.

Test plan

• Read the rendered profile