doc: 6908 add source verification docs v3

[jmtaylor90]

Ticket: #6908

Make sure these boxes are signed before submitting your Pull Request -- thank you.

• [x] I have read the contributing guide lines at https://docs.suricata.io/en/latest/devguide/contributing/contribution-process.html
• [x] I have signed the Open Information Security Foundation contribution agreement at https://suricata.io/about/contribution-agreement/ (note: this is only required once)
• [x] I have updated the user guide (in doc/userguide/) to reflect the changes made (if applicable)
• [ ] I have created a ticket at https://redmine.openinfosecfoundation.org/projects/surica ta/issues (if applicable)

Link to ticket: https://redmine.openinfosecfoundation.org/issues/ https://redmine.openinfosecfoundation.org/issues/6908

Describe changes:

• updated to address review comments in previous PR
• fixed make distcheck failure
• RTD build of this branch - https://rtd-test-suricata.readthedocs.io/en/latest/verifying-source-files.html

Provide values to any of the below to override the defaults.

• To use an LibHTP, Suricata-Verify or Suricata-Update pull request, link to the pull request in the respective _BRANCH variable.
• Leave unused overrides blank or remove.

SV_REPO= SV_BRANCH= SU_REPO= SU_BRANCH= LIBHTP_REPO= LIBHTP_BRANCH=