doc: cross-reference notmuch man pages with actual links

Add internal hyperlink targets for man pages and cross-reference them
using the any role reference. There are a number of alternatives to
accomplish this, but this seems like the combination that retains the
man page section number and the same boldface style in the man pages.

As a bonus, we get sanity checking on the links; for example
notmuch-search-terms.rst had a reference to notmuch-properties(1)
i.e. the wrong section.

The obvious semantic follow-up change would be to only have meaningful
"see also" references instead of having them all everywhere.

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