Resolve ambiguous anchoring in generated RST files

[afsmythe]

While researching https://github.com/votinginfoproject/vip-specification/issues/279 it's become obvious that our RST page anchoring for extended classes could be improved. For two reasons:

• hundreds of Sphinx warnings are generated during make html citing "WARNING: duplicate label < prefix >-< spinal_name >", e.g.

  docs/built_rst/csv/elements/ballot_measure_contest.rst:72: WARNING: duplicate label multi-csv-contest-base, other instance in docs/built_rst/csv/elements/party_contest.rst

• clicking Extends reference links in the documentation will sometimes take the user away from the original class page they are reviewing. For example, while reviewing BallotMeasureContest, if the Extends [ContestBase] link is clicked, the user is taken to the PartyContest page.

Assigning this to the 6.0 Milestone to be a part of the PR for https://github.com/votinginfoproject/vip-specification/issues/279.

This might be improved upon by updating the label value to include the "parent" class when it is passed to https://github.com/votinginfoproject/vip-specification/blob/04116a04136ee3f499a1caf8548d88fba22e45d3/scripts/vippy/rest.py#L131

But this will need some further review :)

[jswiesner]

Do we still intend to address this with 6.0? Or should we move to the backlog?

[afsmythe]

Moved to backlog as it needs some additional review.