Improve config option metadata to define where options can be used and improve JSON schema

[rarkins]

Describe the proposed change(s).

Enhance this source file: https://github.com/renovatebot/renovate/blob/main/lib/config/options/index.ts

Right now we have some options with "parent" defined (such as packageRules>matchPackageNames) but it's not enough info to generate the JSON schema accurately when options such as enabled can be at root or within packageRules, or options such as configMigration must be at the root of config.

It's even more complicated when you consider that we support some dynamic config options such as minor, or manager names such as gomod.

We should brainstorm about all the possible objects which config can be in (root, hostRules, packageRules, managers, etc) and then define the syntax. Perhaps parent should just be an array instead of single string.

[rarkins]

Ref: #26484

[viceice]

as a first step I would suggest to change parent to parents to allow multiple parents.

[rarkins]

I split off #26599 @RahulGautamSingh