Makes README the entrypoint for all CBMC documentation

[feliperodri]

• 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

All modified and coverable lines are covered by tests ✅

Comparison is base (97ab133) 79.08% compared to head (97b9a82) 79.08%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #8102   +/-   ##
========================================
  Coverage    79.08%   79.08%           
========================================
  Files         1698     1698           
  Lines       196457   196457           
========================================
+ Hits        155370   155373    +3     
+ Misses       41087    41084    -3     

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