Help: Add target for policy deprecation link
In the policy pages' "deprecated by definition" box, instead of
linking that phrase to the `cmake-policies(7)` manual page, add
a label (`_cmake-policies-intro`) above the "Introduction" section,
and link to that label using the `:ref:` role instead.
This avoids landing the reader at the `cmake-policies(7)` table of
contents, which is now long enough that it's likely to fill their
entire screen unless they scroll down!
As an added bonus, using `:ref:` instead of `:manual:` avoids
applying monospace formatting to the link text.
FeRD (Frank Dana)