doc: 6908 add tarball verification docs v1

[jmtaylor90]

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• [x] I have read the contributing guide lines at https://docs.suricata.io/en/latest/devguide/contributing/contribution-process.html
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• [x] I have updated the user guide (in doc/userguide/) to reflect the changes made (if applicable)

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

Describe changes:

• add Suricata tarball verification documentation
• RTD build https://rtd-test-suricata.readthedocs.io/en/latest/source-verification.html

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

To use a pull request use a branch name like pr/N where N is the pull request number.

Alternatively, SV_BRANCH may also be a link to an OISF/suricata-verify pull-request.

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

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 83.67%. Comparing base (abb7424) to head (8d069fa).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #11051      +/-   ##
==========================================
+ Coverage   80.63%   83.67%   +3.03%     
==========================================
  Files         922      922              
  Lines      250137   250321     +184     
==========================================
+ Hits       201699   209453    +7754     
+ Misses      48438    40868    -7570     

Flag	Coverage Δ
fuzzcorpus	64.33% <ø> (+0.06%)	:arrow_up:
livemode	18.46% <ø> (-0.10%)	:arrow_down:
suricata-verify	62.76% <ø> (?)
unittests	62.26% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

[jufajardini]

The other failures are related to: https://github.com/OISF/suricata/pull/11041

[jmtaylor90]

continued in https://github.com/OISF/suricata/pull/11111

[jufajardini]

Some of the errors from the CI are doc building or dist check complaints.

/__w/suricata/suricata/doc/userguide/index.rst:6:toctree contains reference to nonexisting document 'source-verification'

(e.g. https://github.com/OISF/suricata/actions/runs/9019965517/job/24784406162?pr=11051#step:11:1108)

My only initial guess is that maybe that empty line before the chapter title makes some distros unhappy? Or there's something else I can't see.

If anyone by any change arrives here, the solution was actually to add the new file to the Makefile.am, as suggested by Jeff (cf https://github.com/OISF/suricata/pull/11111#issuecomment-2127123594)