Add markdown index support

[robtaylor]

This PR adds support for generating a top level index in markdown, similar to the HTML publishing backend.

It also contains adding markdown's requirements into its generated docs, mainly by way of an example of usage - I expect this will need some work if it is desirable!

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 97.14286% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.44%. Comparing base (7865edf) to head (2947607).
Report is 9 commits behind head on develop.

Files with missing lines	Patch %	Lines
doorstop/core/publishers/markdown.py	96.87%	0 Missing and 1 partial ⚠️

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Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #661   +/-   ##
========================================
  Coverage    96.44%   96.44%           
========================================
  Files           40       40           
  Lines         5198     5231   +33     
  Branches      1226     1234    +8     
========================================
+ Hits          5013     5045   +32     
  Misses         110      110           
- Partials        75       76    +1     

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