Strip Provider Prefix from File Names during Migration

[SBGoods]

Closes: #347
Supersedes: #348

Fixes a bug that is encountered when the migrate command and generate command are run on provider documentation that have provider short name prefixes in their filenames (ex. time_rotating.md). The migrate command would move the files to the template folder as-is but the generate command does not recognize these files as valid documentation files because of the prefix, and therefore generates default template for the "missing" resource/data-sources, resulting in duplicate templates. This PR resolves this by introducing a --provider-name flag to the migrate subcommand, and uses the provided value to derive the provider short name and strips the prefix from an file name containing it.

The --provider-name flag is optional for users who do not have the provider prefix in their file names, so they can continue to run the migrate command without any flags, if the defaults are appropriate.

[austinvalle]

LGTM 🚀

[bflad]

Non-blocking: Should migrate --provider-name default to the short name of the provider directory similar to generate --provider-name here?

terraform-plugin-docs/internal/provider/generate.go

Lines 162 to 164 in 836a890

	if g.providerName == "" {
	g.providerName = filepath.Base(g.providerDir)
	}

I think if migrate can do the expected behavior of stripping the prefixes automatically, but potentially leaving an opt-out for developers to skip that default behavior, it could prevent confusion.

[SBGoods]

@bflad Good shout! I completely forgot about the provider directory name. I've updated the code to use it as a default, and tweaked the documentation to make it clear that the prefixes are removed by default.

[bflad]

Looks good to me 🚀

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