docs(mend-hosted): how to perform config validation

[jamietanna]

Changes

It's a somewhat commonly asked question, and has been something I've
been meaning to officially document since mid-November.

Context

Discussions asking about this:

• What is the best way to validate a Renovate config? #30458
• Validating repo config on Mend-hosted apps, when repo is a fork #40375

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• This closes an existing Issue: Closes #
• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

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The public repository:

[setchy]

TIL, this is awesome! Out of interest, is the same true for self-hosted?

[jamietanna]

Yep! it's a nice lesser known feature (that I didn't know about until I started digging into this in ~November)

[jamietanna]

Need to clarify this is base-branch only, not forked PRs