Document steps to perform CMake configuration without network access

[tautschnig]

Build environments may be configured in a way that disallows network
access. The make-based build system already supported this as
downloading and building solvers is a separate step. For CMake, document
the procedure to facilitate this.

Fixes: #6582

• Each commit message has a non-empty body, explaining why the change was made.
• n/a 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).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6649 (b630cc5) into develop (afc53d8) will increase coverage by 0.00%.
The diff coverage is 84.00%.

@@           Coverage Diff            @@
##           develop    #6649   +/-   ##
========================================
  Coverage    76.73%   76.73%           
========================================
  Files         1579     1579           
  Lines       181999   181994    -5     
========================================
- Hits        139652   139650    -2     
+ Misses       42347    42344    -3     

Impacted Files	Coverage Δ
src/goto-programs/slice_global_inits.h	0.00% <0.00%> (ø)
src/goto-programs/string_instrumentation.h	0.00% <0.00%> (ø)
src/goto-programs/restrict_function_pointers.cpp	80.67% <100.00%> (+0.40%)	⬆️
src/memory-analyzer/gdb_api.h	100.00% <100.00%> (+9.52%)	⬆️
src/solvers/smt2/smt2_tokenizer.h	96.96% <100.00%> (ø)
src/util/exception_utils.cpp	97.77% <100.00%> (+3.43%)	⬆️
src/util/exception_utils.h	92.85% <100.00%> (-7.15%)	⬇️
... and 2 more

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[chris-ryder]

Excellent! :-)