docs: add instruction about handling regressions to our core texts for contributors

Add some more detailed instruction on handling regressions to our core texts for contributors, e.g. Documentation/process/6.Followthrough.rst and Documentation/process/submitting-patches.rst. This was asked for at the last Maintainer Summit[1] and is meant to help clarifing what's expected.

The added instructions are somewhat inspired by existing and sometimes similar text in Documentation/process/handling-regressions.rst which is semoved during the series.

The patch-set starts with some basics, which are based on the existing text or recent statements from Linus. I tried to stay close to what was said, but used my own words here -- if I got something wrong just tell me so I can fix this.

The guidelines then are extended in multiple steps to cover more things and be more specific in some areas. Some of that is meant to work against problems I noticed during my regression tracking work. Most of these are my words. These thus need to be discussed properly and in the end ideally ACKed by Linus, so we all know what he wants. The parts I expect to get more discussion are found near the end of the patchset.

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v1:
- initial version

Thorsten Leemhuis (6):
docs: more detailed instructions on handling regressions
docs: 6.Followthrough.rst: when to involved Linus in regressions
docs: 6.Followthrough.rst: interaction with stable wrt to regressions
docs: 6.Followthrough.rst: tags to use in regressions fixes
docs: 6.Followthrough.rst: more specific advice on fixing regressions
docs: 6.Followthrough.rst: advice on handling regressions fixes

 Documentation/process/6.Followthrough.rst     | 152 ++++++++++++++++--
 .../process/handling-regressions.rst          | 111 +------------
 Documentation/process/submitting-patches.rst  |  20 +++
 3 files changed, 157 insertions(+), 126 deletions(-)

base-commit: 010a08cbf0d63b246c4cd0d385c3f664add2322d

2.45.0