QA/Markdown CS: use consistent header styles

* Always use the `atx` style headers (`# ...`).
* Use consistent blank lines around headers.
* No trailing punctuation in headers.
* Every file should have a header.
* First header in a file should be an H1.
   Note: the issue templates do not trigger this error as they use frontmatter with a `title` attribute. However, frontmatter is not supported for pull request templates.

Author: jrfnl

.github/CONTRIBUTING.md

@@ -1,5 +1,4 @@
-Contributing
--------------
+# Contributing
 
 Thank you for your interest in contributing to PHP_CodeSniffer!
 
@@ -71,7 +70,7 @@ This includes checking whether the bug is something which should be fixed in **_
 To find bugs which need triage, look for issues and PRs with the
 ["Status: triage"](https://github.com/PHPCSStandards/PHP_CodeSniffer/labels/Status%3A%20triage) label.
 
-#### Typical bug triage tasks:
+#### Typical bug triage tasks
 * Verify whether the bug is reproducible with the given information.
 * Ask for additional information if it is not.
 * If you find the issue is reported to the wrong repo, ask the reporter to report it to the correct external standard repo
@@ -95,7 +94,7 @@ To get access to a PHPCS version which includes the patch from a pull request, y
     The PHAR files can be found on the summary page of the test workflow run for the PR.
     If the workflow has not been run (yet), the PHAR artifact may not be available (yet).
 
-#### Typical test tasks:
+#### Typical test tasks
 * Verify that the patch solves the originally reported problem.
 * Verify that the tests added in the PR fail without the fix and pass with the fix.
 * For a fix for false negatives: verify that the correct error message(s) are thrown by the patched code.

.github/ISSUE_TEMPLATE/bug_report.md

@@ -19,6 +19,7 @@ standard repository (not here).
 -->
 
 ## Describe the bug
+
 A clear and concise description of what the bug is.
 
 ### Code sample
@@ -44,6 +45,7 @@ PHPCS output here
 ```
 
 ## Expected behavior
+
 A clear and concise description of what you expected to happen.
 
 ## Versions (please complete the following information)
@@ -57,9 +59,10 @@ A clear and concise description of what you expected to happen.
 | Install type | [e.g. Composer (global/local), PHAR, git clone, other (please expand)]
 
 ## Additional context
+
 Add any other context about the problem here.
 
-## Please confirm:
+## Please confirm
 
 - [ ] I have searched the issue list and am not opening a duplicate issue.
 - [ ] I confirm that this bug is a bug in PHP_CodeSniffer and not in one of the external standards.

.github/community-cc-list.md

@@ -1,6 +1,6 @@
-<!--
-PHP_CodeSniffer Community CC List
+# PHP_CodeSniffer Community CC List
 
+<!--
 This list will be used on select tickets to ping the wider PHP_CodeSniffer community for input.
 
 If you want to be on this list, feel free to submit a PR to add yourself.

.github/pull_request_template.md

@@ -4,7 +4,7 @@
 Please target the `master` branch when submitting your pull request, unless your change **only** applies to PHPCS 4.x.
 -->
 
-## Description
+# Description
 <!--
 What do you want to achieve with this PR? Why did you write this code? What problem does this PR solve?
 Describe your changes in detail and, if relevant, explain which choices you have made and why.