[ENH] Add a separate documentation for pytorch-forecasting v1 and v2

[phoeenniixx]

Reference Issues/PRs

Fixes parts of #2277 (separation of v1 and v2)

What does this implement/fix? Explain your changes.

This PR create separate header for v2 documentation and creates clear distinction between both the versions and adds clearer api references

checklist for this PR

• Add some references for v2 to readme as well
• Create a new header for v2 API
• Add API references
• separate v2 models and datamodules from v1
• Clearly mention that metrics are still the same as v1
• make the models table automated and add new models which are not present. Waiting for [ENH] auto-generate model overview table from registry tags #2236 to complete
• Add separate tutorials for v2 - should be easily distinguishable from v1

[felipeangelimvieira]

One possibility: use gh-pages and dump documentation to specific directories for each version. Example:
https://github.com/felipeangelimvieira/prophetverse/tree/gh-pages

There is a html at index that redirects to the latest version. You can then add a custom domain.

[phoeenniixx]

Thanks a lot for sharing this! I will look into it

[felipeangelimvieira]

There's a CI that dumps the doc files when there's a release. You can also manually edit gh-pages branch.