docs: Fix RTD build, remove exhale

[paulgessinger]

This removes Exhale from the build, and directly uses breathe.apidoc
to generate symbol listings. They are less fancy, but appear to be more
robust.

[codecov]

Codecov Report

Merging #949 (aad855b) into main (4452021) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main     #949   +/-   ##
=======================================
  Coverage   48.48%   48.48%           
=======================================
  Files         334      334           
  Lines       17056    17056           
  Branches     8067     8067           
=======================================
  Hits         8270     8270           
  Misses       3085     3085           
  Partials     5701     5701           

Impacted Files	Coverage Δ
...ore/include/Acts/Material/ProtoSurfaceMaterial.hpp	27.27% <ø> (ø)
Core/include/Acts/Vertexing/LinearizedTrack.hpp	75.00% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4452021...aad855b. Read the comment docs.

[paulgessinger]

Demo build on RTD is here: https://acts.readthedocs.io/en/docs-testing/

[HadrienG2]

Looks good to me.