Feature/devops ci

[memsharded]

Very initial draft of the first sections, to get team early feedback.

It is better reviewed generating the html docs, not in code.

Depends on https://github.com/conan-io/examples2/pull/153

Close https://github.com/conan-io/docs/issues/3849 Close https://github.com/conan-io/conan/issues/3003 Close https://github.com/conan-io/conan/issues/6589 Close https://github.com/conan-io/conan/issues/6045 Close https://github.com/conan-io/conan/issues/8587 Close https://github.com/conan-io/conan/issues/16309

[memsharded]

Thanks very much for all the feedback!

As this PR is very large and with deep topics I'll suggest the following:

• Keep this PR reviews exclusively to the contents of the PR itself: if something doesn't work, the examples code in the examples Github repo is not matching the docs, the explanations are unclear, the graphics are poor, typos, grammar, etc., etc.
• Keep in mind that this section is "didactical", it doesn't aim to be a full and exhaustive implementation of a unique way of implementing CI, but rather to provide some basic concepts, tools and practices, to allow users to implement their own.
• This is the initial "abstract/core" part of the CI tutorial. Other new sections will follow, hopefully diving deeper and deeper, from explaining different branching/merging/integration strategies, to some implementation examples in a real CI.
• For other CI related topics and questions that this PR could spark, it would be better to have a dedicated new issue for it, referencing this one, as it is likely that they could result in future PRs adding more info to this current proposal, but it is not planned to augment or increase much more this initial proposal, otherwise it would be intractable.

Many thanks!