[doc] Move webinars and tutorials under the same section

[codecov[bot]]

Codecov Report

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

Comparison is base (cc38b84) 86.71% compared to head (d40f13f) 86.70%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3047 +/- ## ========================================== - Coverage 86.71% 86.70% -0.01% ========================================== Files 61 61 Lines 12002 12002 ========================================== - Hits 10407 10406 -1 - Misses 1595 1596 +1 ```

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