[MRG] Add simple example to documentation and readme

[rflamary]

• Adds a simple example file with how to use skada fro beginers
• Add some code examples in the redame from this example file

[codecov[bot]]

Codecov Report

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

Comparison is base (cb30c6d) 88.95% compared to head (2cc2fe5) 88.95%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #99 +/- ## ========================================== - Coverage 88.95% 88.95% -0.01% ========================================== Files 41 41 Lines 2708 2707 -1 ========================================== - Hits 2409 2408 -1 Misses 299 299 ```

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