Bug 1849943: Remove BMO references and mod_perl/CGI install instructions

[topunix]

Part 2 of Bug 1849943. Follows #190 (Apache and Nginx reverse proxy pages). No file overlap, so they can merge in either order.

Commit 1: Remove BMO-specific references

• Evaluating/test links pointed at Mozilla's BMO test server. They now point at running a local copy via Docker (about/index.rst, using/creating-an-account.rst, using/filing.rst, using/finding.rst).
• Getting help: the Mozilla forum and the mozilla.support.bugzilla Google Group are gone. Now points at https://www.bugzilla.org/support/.
• Example hostnames switched from bugzilla.mozilla.org / bugzilla.allizom.org to bugzilla.example.org (integrating/auth-delegation.rst, integrating/auth0.rst, api/core/v1/flag-activity.rst).
• api/core/v1/general.rst: "future versions of BMO" now reads "future versions of Bugzilla".
• using/finding.rst: hardcoded BMO permissions URL replaced with a Preferences > Permissions reference.
• integrating/skins.rst: removed the dead git.mozilla.org link. Also corrected "ships with Classic and Dusk": Harmony ships only the default skin, and Skin.pm still loads extras from skins/contrib.
• integrating/templates.rst: removed the BMO guided-form link (the templates still ship in core) and fixed the filename (comment-guided.txt.tmpl, not .html.tmpl).
• integrating/auth-delegation.rst: fixed the nested list indentation behind the two existing Sphinx warnings, plus two typos.

Intentionally kept: style.rst (how to link BMO bugs), the "file a docs bug" link in about/index.rst, and the upstream bug link in administering/workflow.rst. Bugzilla's own bugs still live on BMO.

Commit 2: Remove mod_perl and CGI web server instructions

• linux.rst: dropped mod_perl, mod_perl-devel, httpd-devel, perl(mod_perl2), and libapache2-mod-perl2(-dev) from the package lists.
• windows.rst, mac-os-x.rst: replaced the "any web server that can run CGI" sections with a pointer to the reverse proxy pages. Removed the DocumentRoot symlink / Web Sharing steps (macOS) and the httpd.conf / ScriptInterpreterSource troubleshooting (Windows).
• oracle.rst: ORACLE_HOME and LD_LIBRARY_PATH are now set on the app process instead of via SetEnv in httpd.conf.
• multiple-bugzillas.rst: SetEnv PROJECT in an Apache vhost no longer reaches the app. Nothing in bugzilla.pl or Bugzilla::App sets PROJECT per request, so each instance now runs as its own process on its own port behind the proxy.

Not in this PR

• windows.rst and mac-os-x.rst still describe older setups (OS X 10.7 with Perl 5.12, Strawberry Perl module lists), and apache-windows.rst / iis.rst are still stubs. These need a full rewrite rather than corrections, so the "being revised" warning in installing/index.rst is left in place.
• Recommended hardware guidance (comment 3 on the bug) is new content and not covered here.

Testing

Full rebuild with the pinned docs/requirements.txt versions (sphinx-build -E). The only warning left is the missing installing/nginx toctree entry, which the part 1 PR adds. The extension glob warnings are pre-existing and come from building without makedocs.pl.