Extend OpenAPI/spec description

[CasperWA]

This adds a notion of the spec's source material to the description.
The extension comes from a suggestion to the PR Materials-Consortia/OPTIMADE#242

[codecov]

Codecov Report

Merging #151 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master    #151   +/-   ##
======================================
  Coverage    85.8%   85.8%           
======================================
  Files          39      39           
  Lines        1831    1831           
======================================
  Hits         1571    1571           
  Misses        260     260

Flag	Coverage Δ
#unittests	85.8% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
optimade/server/main.py	89.09% <100%> (ø)	⬆️
optimade/server/main_index.py	86.36% <100%> (ø)	⬆️

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[merkys]

Very good - now we have the provenance record inside the generated schema!

[CasperWA]

Is there anything else you have in mind that could/should be added to the spec description now that we're at it @merkys? :)

[merkys]

Is there anything else you have in mind that could/should be added to the spec description now that we're at it @merkys? :)

I guess it's fine as is now, as it mentions the version of OPTiMaDe specification, inputs and tools to build it.