Implement migrate sub-command

[SBGoods]

Implements a new migrate subcommand that migrates existing provider docs using the old website directory structure (website/docs/) to a terraform-plugin-docs-supported templates directory.

The migrate subcommand takes the following actions:

• Copies the --old-website-source-dir folder to the --tempates-dir folder (will create this folder if it doesn't exist)
• Renames docs/d/ and docs/r/ subdirectories to data-sources/ and resources/ respectively
• Change file suffixes for template files from .html.markdown to .md.tmpl
• Extracts code blocks from website docs to create individual example files in --examples-dir (will create this folder if it doesn't exist)
• replace extracted example code in website templates with tfplugindocs template code referring to example files.
• Copies non-template files to --templates-dir folder

[bflad]

Overall this is looking pretty good! Some high level comments/questions and then its probably good to go! 👍

[bflad]

I do also wonder if we should have a flag (or bake the behavior in) to remove the old website/docs/... files. I presume the developer workflow for this command is a one-time operation that would be submitted as a pull request and removing the old files would save a lot of time. For additional context, the public registry does not support having both website/docs/... and docs/... layouts at the same time -- it will take precedence with the legacy file layout over the newer layout (for whatever reason), so its important to get rid of the old files to prevent developer confusion.

[austinvalle]

This is awesome @SBGoods! I left some random thoughts/questions but everything LGTM

[austinvalle]

At first glance, it wasn't immediately obvious what the input directories were vs. the output directory without reading the help text. Would it be confusing/misleading to name the --templates-dir as --output-dir?

[SBGoods]

I think the name should be the same to keep it in parity with the generate command with the same flag. If we changed the flag to something like --output-dir it might not be obvious to the user that they would need to specify generate --template-dir to that directory after migration.

[austinvalle]

Ah okay that's helpful context! Makes sense to me 👍🏻

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