Add alternative software

[sappelhoff]

closes #84

see: https://pybv--85.org.readthedocs.build/en/85/#alternatives

[codecov]

Codecov Report

Merging #85 (d02ed98) into main (2740359) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main       #85   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines          431       431           
=========================================
  Hits           431       431           

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[cbrnr]

LGTM. It would be nice if the table had separate columns for e.g. read support, write support, open source – but it looks like there is not enough horizontal space in the docs layout, so the current version is completely fine!