Move legacy guides to a separate repository

[aldesantis]

Description

The legacy guides have been moved to https://github.com/solidusio/legacy-guides, because they are going to be replaced by the new guides hosted at https://github.com/solidusio/edgeguides pretty soon.

The https://guides.solidus.io site has been updated to point to the new repository.

Checklist:

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message
• I have updated Guides and README accordingly to this change (if needed)
• I have added tests to cover this change (if needed)
• I have attached screenshots to this PR for visual changes (if needed)

[aldesantis]

Please note that Netlify checks are only failing because I've moved the build to Cloudflare Pages (since that's where we're managing DNS for solidus.io nowadays).

[waiting-for-dev]

Lovely! ❤️

We can use this PR to remove a couple of references to the old guides in /guides:

1. Stoping CI for guides-only changes.
2. Ignoring node_modules directory within guides for rubocop.

For 1., we might want to keep it if the new guides will be also placed in guides, although it has some problems (like stopping a build for a multi-commit PR that modifies more than the guides but only the guides in the last commit).

[aldesantis]

@waiting-for-dev I've addressed your suggested changes, and I have also taken the chance to update the PR template: it now points to the new guides repository.

As for the CI, I have decided against keeping the documentation in this repo: because the new guides are versioned, and the content for all versions is in the same branch (just in different directories), it would lead to a very weird situation where we have the documentation for all Solidus versions in the master branch of solidusio/solidus, which would make it weird to navigate the documentation from GitHub and/or to update the documentation for older versions.

I have explored the possibility of using the solidusio/edgeguides repo just to host the Docusaurus app, and have it read the actual content of the guides from the different branches in solidusio/solidus. It works, but it creates a bunch of additional issues/gotchas with deployments and preview environments. I think it's just too big of a headache at this point, and we can explore alternative solutions in the future.

[aldesantis]

CI failure seems unrelated, merging.