docs: fix line breaks around elements in markdown

Ensure consistent line breaks around headings, lists and code blocks.

To make them more readable and also more portable (which could help when
previewing/converting files locally).

From the CommonMark Spec (version 0.30, published on 2021-06-19)[1]:

> 1.2 Why is a spec needed?

[...]

> 2. Is a blank line needed before a block quote or heading?  Most
>    implementations do not require the blank line.  However, this can
>    lead to unexpected results in hard-wrapped text, and also to
>    ambiguities in parsing (note that some implementations put the
>    heading inside the blockquote, while others do not).  (John Gruber
>    has also spoken in favor of requiring the blank lines.)[2]
>
> 3. Is a blank line needed before an indented code block?
>    (`Markdown.pl` requires it, but this is not mentioned in the
>    documentation, and some implementations do not require it.)

Fixes[3]:

* MD022 Headers should be surrounded by blank lines
* MD031 Fenced code blocks should be surrounded by blank lines
* MD032 Lists should be surrounded by blank lines

Environment: ruby-mdl 0.12.0-1 on Artix Linux.

[1] https://spec.commonmark.org/0.30/#why-is-a-spec-needed-
[2] http://article.gmane.org/gmane.text.markdown.general/2146
[3] https://github.com/markdownlint/markdownlint/blob/main/docs/RULES.md

CONTRIBUTING.md

@@ -1,6 +1,7 @@
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+
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@@ -9,6 +10,7 @@ our [dedicated issue](https://github.com/netblue30/firejail/issues/1139).
 
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+
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@@ -28,6 +30,7 @@ We take security bugs very seriously. If you believe you have found one, please
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