Docs: Refactored the documentation and deprecation warnings

[cwognum]

Changelogs

• Updated the documentation to reflect recent changes (e.g. DatasetV2 and BenchmarkV2)
• Throw deprecation warnings for some old classes and methods we plan to phase out.

---

Checklist:

• Was this PR discussed in an issue? It is recommended to first discuss a new feature into a GitHub issue before opening a PR.
• Add tests to cover the fixed bug(s) or the newly introduced feature(s) (if appropriate).
• Update the API documentation if a new function is added, or an existing one is deleted.
• Write concise and explanatory changelogs above.
• If possible, assign one of the following labels to the PR: feature, fix, chore, documentation or test (or ask a maintainer to do it for you).

---

Some of our recent changes, e.g. DatasetV2 and BenchmarkV2, haven't been well documented. This was putting a large burden on myself to be very hands-on in the creation of new datasets and benchmarks. This PR drastically refactors the documentation to reflect recent changes and adds deprecation warnings for outdated pieces of code we want to phase out.

[zhu0619]

Wow, what a big change! Good timing for me to learn all the new APIs.
The tutorials are particularly well done – clear, concise, and easy to follow. Fantastic work!