add extension point to add scripts at the end of html page

[SebastienAndreo]

This is an enhancement or feature.

Summary

I have created an overridable file myscripts.html and include it at the end of default.html to be able to control more accurately some JS execution. I also made the customization point more visible by creating a folder custom in the _includes folder.

Context

It is sometimes useful to control the execution of some javascript being able to add them at
the end helped me to have the right rendering.

[iBug]

Consider taking a look at #2116 and #2613 first. I think it's the recommended way to override scripts.html directly, but I could be wrong.

[tsmanner]

This sounds like a use for the after_footer_scripts config option. They're added at the end of scripts.html, which I think is equivalent to what this PR does, if the intent is to add <script src="/path/to/some/script.js"></script> tags to the site.

[github-actions]

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