Add the docs for secrets masking + set default masks for AWS

[arm4b]

Closes #374

Also adds AWS secret ENVs to the default config, so it's secure by default.

[arm4b]

Tested the regex rules for AWS locally.

But it's also interesting that you can have unit tests and examples for the regex rule at https://regex101.com/r/snz1j6/1

[LeoDiazL]

I would move that URL to the top of the regex comments, or to the bottom, so that every regex keeps the same style of #comment / replace

[arm4b]

Sorry, the PR was auto-merged.

Though the URL was specific to the AWS_ACCESS_KEY_ID rule block as an example, moving it more to the top of the secrets section would be less relevant.

[LeoDiazL]

Just to be consistent, I would use

# comment 
replace
# comment
replace

We could add a bigger headder on top of the replacements with the URL and some info if needed. Or at the bottom.

[LeoDiazL]

I would move that URL to the top of the regex comments, or to the bottom, so that every regex keeps the same style of #comment / replace