Add ClassifAI v3 migration guide

[iamdharmesh]

Description of the Change

PR adds v3 migration guide docs to help users to upgrade to classifAI v3.

Closes #674

How to test the Change

Changelog Entry

Added - Migration guide for upgrading from v2 to v3

Credits

Props @iamdharmesh

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[dkotter]

@iamdharmesh I've made a handful of changes, most importantly I've added some snippets that show how a third-party developer could add new Features or Providers. In putting this together, I found a few issues with the way we register those that was preventing that from working, so that has been fixed in this PR as well.

[iamdharmesh]

Thanks for making changes @dkotter. All looks good to me. The table layout looks like this to me in Chrome browser, may need some CSS updates. is it same for you? Thank you.

[dkotter]

Thanks for making changes @dkotter. All looks good to me. The table layout looks like this to me in Chrome browser, may need some CSS updates. is it same for you? Thank you.

Yeah, I added overflow: scroll on the table cells because otherwise the table was overlapping the navigation menu on the right. I'm open to other suggestions on how best to handle this but personally I'm fine with the scrolling

[jeffpaul]

Agreed, for now, that scrolling cell is acceptable for a dev docs site.