Retitle "Metadata syntax reference" to "Custom action syntax reference"

[nickwalkmsft]

Code of Conduct

• I have read and agree to the GitHub Docs project's Code of Conduct

What article on docs.github.com is affected?

Metadata syntax reference

What part(s) of the article would you like to see updated?

Suggesting retitling to "Custom action syntax reference", with corresponding updates to the URL slug and TOC entry.

Additional information

I initially wasn't able to find this information in the docs due to the article title and TOC entry.

[welcome]

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[Sharra-writes]

@nickwalkmsft Thanks for opening an issue! I'll get this triaged for review.