ci: Add pre-commit hooks for basic formatting + gitleaks

[MoritzWeber0]

Description

Add pre-commit hooks:

• Gitleaks to identify secrets before pushing them
• Check for large files (to avoid a large repository)
• Check for invalid file structures
• Add new line to the end of each file (to avoid changes to the last line when content is added to a file)
• Remove trailing whitespaces. Since sometimes a double whitespace was used for a new line, I changed it to \ for new lines. This is a lot more readable (whitespaces are usually not displayed in default editor setups)
• actionlint to check for valid Github Actions files

See #80

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[therobrob]

Sounds good.
Two Questions:

• what´s the limitation for the file size? Do we need to specify a value here?
• Is it mandatory to use a \ for a newline? I think it's a bit counterintuitive for contributors, isn´t it?

[MoritzWeber0]

what´s the limitation for the file size? Do we need to specify a value here?

The default is 500kB, but we can also change it to a different value.

Is it mandatory to use a \ for a newline? I think it's a bit counterintuitive for contributors, isn´t it?

That's the CommonMark syntax, either <br />, two new lines, \ or two whitespaces. There are reasons to avoid the two whitespaces because it's hard to read (as mentioned above, default editors don't show trailing whitespaces, and in reviews it's also hard to overlook them).

The most known markdown guide also recommends to avoid double-whitespaces as line-breakers:
https://www.markdownguide.org/basic-syntax/#line-break-best-practices

You can use two or more spaces (commonly referred to as “trailing whitespace”) for line breaks in nearly every Markdown application, but it’s controversial. It’s hard to see trailing whitespace in an editor, and many people accidentally or intentionally put two spaces after every sentence. For this reason, you may want to use something other than trailing whitespace for line breaks.

To be fair, I don't expect non-tech contributors to run the pre-commit hooks, it's also not part of the content contribution guide. That's something we as maintainers have to do in the end.