Extend git flow complex example to post merge and rebase behaviour

[9swampy]

Description

Extend the GitFlowComplexExample to document unexpected post merge and rebase behaviour. Fix the integration test fixtures so they can accept a "master" main branch instead of hardcoding "main".

Related Issue

Motivation and Context

I have been really struggling to get GitVersion to work post the v6 changes so I checked out the repo to run through sample scenarios to try and adapt but I either need more documentation to understand what the new behaviour is or the new behaviour is possibly unintended.

Rather than just rely on the Integration tests with injected configuration, I wanted to be able to revisit the repository that is created by the integration test and I observe

1. Inconsistent behaviour between the TestFixture and when running dotnet-gitversion against GitVersion.yml output by the test fixture (problem number 1 documented in the penultimate commit in the amended GitFlowComplexExample - I set this assertion to pass but behaviour in the test does not match behaviour observed with dotnet-gitversion 6.3.0 and a persisted GitVersion.yml)
2. Consistent but unanticipated behaviour on a rebase (problem number 2 which is the current failure in the amended GitFlowComplexExample)

How Has This Been Tested?

GitFlowComplexExample

Screenshots (if appropriate):

Checklist:

This is a WIP proposal to document a problem & ask for help; if I get direction I'm happy to update the PR as offered/advised.

• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have added tests to cover my changes.
• [ ] All new and existing tests passed.