Add `etna.datasets.load_dataset` to public API

[ArtemLiA]

Before submitting (must do checklist)

• [x] Did you read the contribution guide?
• [x] Did you update the docs? We use Numpy format for all the methods and classes.
• [x] Did you write any new necessary tests?
• [x] Did you update the CHANGELOG?

Proposed Changes

• Make in a "User guide" page link to load_dataset, it will allow users to check the parameters
• Make in load_dataset reference to "User guide" page for users to see the list of all available datasets.
• Add load_dataset to datasets.rst file.
• Little fix of docstring in load_dataset

Closing issues

Closes #465

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.60%. Comparing base (0f98ec0) to head (87363cd). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #484 +/- ## ======================================= Coverage 90.60% 90.60% ======================================= Files 247 247 Lines 16481 16481 ======================================= Hits 14933 14933 Misses 1548 1548 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.