QA/Markdown CS: various tweaks

* Code snippets which either _look like markdown_ or could confuse markdown parsers, should be wrapped in backticks.
* Text in square brackets, which is not a link, should escape the square brackets.
* Various (tooling) names should use proper case, except when used in code snippets.
    To that end, either fix the case of the term or wrap the code snippet in backticks.
* No blank line within lists.
* No trailing spaces.

Note: especially in the CHANGELOG file, there is a lot more text which should be wrapped in backticks, but that's outside the scope of this current PR, which is aimed at one the one hand preventing markdown parsing problems and on the other hand allowing to run an automated Markdownlint check via CI.

Author: jrfnl

.github/CONTRIBUTING.md

@@ -339,7 +339,7 @@ While not strictly required, it is greatly appreciated if you comply with the fo
 
 * Prefix the commit short description with a hint as to what code is touched in the commit.
     Example: If you have a bug fix for the `Squiz.WhiteSpace.OperatorSpacing` sniff, it is a good idea to prefix
-    the short description with `Squiz/OperatorSpacing: `.
+    the short description with `Squiz/OperatorSpacing:`.
     Another example: if your PR addresses an issue with the Filter classes, prefix the short description
     with `Filters:` or `Filters/FilterName:`.
     Doing so will:

.github/pull_request_template.md

@@ -39,7 +39,7 @@ Fixes #
 - [ ] I grant the project the right to include and distribute the code under the BSD-3-Clause license (and I have the right to grant these rights).
 - [ ] I have added tests to cover my changes.
 - [ ] I have verified that the code complies with the projects coding standards.
-- [ ] [Required for new sniffs] I have added XML documentation for the sniff.
+- [ ] \[Required for new sniffs\] I have added XML documentation for the sniff.
 
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