feature #15081 [Runtime] Init Runtime component (Nyholm, wouterj)

This PR was merged into the 5.3-dev branch.

Discussion
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[Runtime] Init Runtime component

This is documentation for symfony/symfony#38465

The docs is focus on "how to use" instead of "how it works". I will add a "creating your own Runtime" which will include "how it works".

As you may notice. I use two concepts: "the callback" and "the application". The fact that "the application" can be a callback and will be wrapped in callbacks is irrelevant here. I found it much easier to understand this way.

Commits
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ef5f04b Apply suggestions from code review
76de973 [#15081] Finish the new Runtime docs
e7bacf1 Apply suggestions from code review
06414b7 Apply suggestions from code review
7388cc9 Init Runtime docs

Author: wouterj

.doctor-rst.yaml

@@ -101,3 +101,4 @@ whitelist:
         - 'provides a ``loginUser()`` method to simulate logging in in your functional'
         - '.. code-block:: twig'
         - '.. versionadded:: 3.6' # MonologBundle
+        - '// bin/console'