docs: auto-generate config filenames

[jamietanna]

Changes

As noted in #40608, we're currently showing an outdated list of possible
configuration filenames.

We can instead automate this by using the known configuration files we
know, and all platform-specific filenames.

We should also be more explicit that the Platform-specific filenames are
only followed when running on that Platform.

Context

Please select one of the below:

• This closes an existing Issue: Closes Configuration filename options are outdated #40608
• 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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Documentation (please check one with an [x])

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I have verified these changes via:

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• No unit tests but ran on a real repository, or
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The public repository:

[jamietanna]

You can store your Renovate configuration file in one of these locations:

1. `renovate.json`
1. `renovate.json5`
1. `.github/renovate.json`
1. `.github/renovate.json5`
1. `.gitlab/renovate.json`
1. `.gitlab/renovate.json5`
1. `.renovaterc`
1. `.renovaterc.json`
1. `.renovaterc.json5`
1. `package.json` _(within a `"renovate"` section)_

Or in a custom file present within the [`configFileNames`](./self-hosted-configuration.md#configfilenames).
The bot first checks all the files in the `configFileNames` array before checking from the above file list.

[viceice]

are we sure tjie platform config files are already fully implemented? I remember the we maybe didn't pass the platform parameters yet 🤔

[jamietanna]

@RahulGautamSingh would you be able to double check, please? 🙏

[jamietanna]

Related: #10682 #10651

[RahulGautamSingh]

We need to pass platform here: https://github.com/renovatebot/renovate/blob/main/lib/workers/repository/init/merge.ts#L49

[viceice]

we should exclude it for now, as we don't use the argument yet

[jamietanna]

OK, so this change was off the back of #40608 but maybe I've misunderstood as I thought that #40065 was actually being used, but it looks like it's not until #10682 #10651 ?

Right now, we can merge this in, and the output will be effectively what we've got (but less likely to go out of date)

However, until #10682 #10651 the changes in this PR will slightly be useful, but not super helpful