Add new documentation for homebrew tap

[NlightNFotis]

This PR adds new documentation for both users and developers of the CBMC tap.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6817 (cd4139b) into develop (7f0f486) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head cd4139b differs from pull request most recent head 761f7fb. Consider uploading reports for the commit 761f7fb to get more accurate results

@@            Coverage Diff            @@
##           develop    #6817    +/-   ##
=========================================
  Coverage    77.00%   77.01%            
=========================================
  Files         1594     1594            
  Lines       184359   184954   +595     
=========================================
+ Hits        141966   142438   +472     
- Misses       42393    42516   +123     

Impacted Files	Coverage Δ
...oto-instrument/contracts/instrument_spec_assigns.h	94.73% <ø> (ø)
src/goto-instrument/contracts/contracts.cpp	94.48% <100.00%> (+0.59%)	⬆️
...o-instrument/contracts/instrument_spec_assigns.cpp	97.42% <100.00%> (+0.28%)	⬆️
src/goto-instrument/contracts/utils.h	83.01% <0.00%> (-16.99%)	⬇️
src/solvers/flattening/boolbv.cpp	80.33% <0.00%> (-0.06%)	⬇️
src/util/std_expr.h	93.18% <0.00%> (ø)
src/util/replace_symbol.h	100.00% <0.00%> (ø)
src/util/replace_symbol.cpp	88.29% <0.00%> (+0.34%)	⬆️
... and 6 more

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[TGWDB]

Note that this partially fixes #6750

[TGWDB]

Looks good.

[thomasspriggs]

Under what circumstances do we need to create a tap locally? Is this part of creating an entirely separate tap on github, or modifying the tap that already exists to add new versions?

[NlightNFotis]

Yeah that was there just for documentation purposes.

Most likely you won't need to do that

It's a one off for setting up a new tap, and as we already have one, (it's highly likely) no one's going to need to do that again.