Markdown docs for runtime assertions [#3636642]

Problem/Motivation

Steps to reproduce

Proposed resolution

Create new documentation using the runtime assertion section from core.api.php and the Runtime assertion API docs on d.o.
Delete the runtime assertion docs from d.op

Remaining tasks

Review
Change link in example.settings.local.php

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet

Issue fork drupal-3636642

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3636642--markdown-docs changes, plain diff MR !17747
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