Add symtab2gb man page

[tautschnig]

This was built using help2man and then manually expanded.

• 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.
• n/a 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 #6942 (99f5075) into develop (40bd2b5) will not change coverage.
The diff coverage is n/a.

❗ Current head 99f5075 differs from pull request most recent head 85a6675. Consider uploading reports for the commit 85a6675 to get more accurate results

@@           Coverage Diff            @@
##           develop    #6942   +/-   ##
========================================
  Coverage    77.79%   77.79%           
========================================
  Files         1568     1568           
  Lines       180329   180329           
========================================
  Hits        140290   140290           
  Misses       40039    40039           

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