docs: Add docs clean target to cmake

[paulgessinger]

This PR adds a target docs-clean to CMake which will clean out all files generated by the docs build.

[AJPfleger]

lgtm. thanks

[andiwand]

should we include a dependency from docs to docs-clean so we always get a fresh build?

[codecov]

Codecov Report

Merging #1393 (e2d1e3c) into main (ba1e563) will not change coverage.
The diff coverage is n/a.

❗ Current head e2d1e3c differs from pull request most recent head 756171b. Consider uploading reports for the commit 756171b to get more accurate results

@@           Coverage Diff           @@
##             main    #1393   +/-   ##
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  Coverage   47.76%   47.76%           
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  Files         380      380           
  Lines       20164    20164           
  Branches     9387     9387           
=======================================
  Hits         9632     9632           
  Misses       4083     4083           
  Partials     6449     6449           

Impacted Files	Coverage Δ
...clude/Acts/Vertexing/AdaptiveMultiVertexFitter.ipp	46.00% <0.00%> (ø)

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[paulgessinger]

should we include a dependency from docs to docs-clean so we always get a fresh build?

Not sure. That would take longer to run every time.