Source build documentation should have comments about the spec files and the debian packaging files

[cmcantalupo]

estimate 1

• The modification is to https://geopm.github.io/build.html#source-builds
• The change is an addition: document the files that control the packaging
• The reason for the change is that it can be confusing that options passed to configure do not propagate to the configure command used in packaging i.e. in the geopm-*.spec or debian/rules files.
• The documentation component to change is
  • https://github.com/geopm/geopm/blob/dev/service/docs/source/build.rst#building-geopm-service-rpms
  • https://github.com/geopm/geopm/blob/dev/service/docs/source/build.rst#building-geopm-service-debian-packages