Adding a META tag to the doc tree for Google Analytics/Bing

The only way we can track 404's on ReadTheDocs is with Google Search console (and the related BING tools, which validate using google, ie if you access to a thing in the google console, bing will also give you access to their tools with no extra work.

We cannot use the "upload an HTML file" to RTD, nor a TXT record, and the Google Analytics tag method isn't compatible. The META tag is pretty much the only option that works here.

Google Search Console wants us to add:

<meta name="google-site-verification" content="0-DMrB_qDynsvXBKJhpsS5m8l5oVea8Qe2ojkudjtCY" /> to our default page (presently bind 9.18.16)

My reading of Sphinx implies that we need to add:

.. meta::
   :google-site-verification: 0-DMrB_qDynsvXBKJhpsS5m8l5oVea8Qe2ojkudjtCY

to doc/arm/index.rst to cause this meta tag to be added. It's fine if this gets committed now and has to wait for the next release before it shows up in tree, i.e. for the release cut of 9.18.17