Examples for xAOD

[gordonwatts]

• Example that uses the internal xAOD typing that is done by default in the xAOD backend (there isn't much!).
• Simple example that uses the typed interface to access jets.
• Added changes to examples.rst to reference these examples.

Fixes #454

[codecov[bot]]

Codecov Report

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

Project coverage is 82.28%. Comparing base (989c7bb) to head (802caaa).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## doc_week #455 +/- ## ========================================= Coverage 82.28% 82.28% ========================================= Files 26 26 Lines 1383 1383 ========================================= Hits 1138 1138 Misses 245 245 ``` | [Flag](https://app.codecov.io/gh/ssl-hep/ServiceX_frontend/pull/455/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ssl-hep) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/ssl-hep/ServiceX_frontend/pull/455/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ssl-hep) | `82.28% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=ssl-hep#carryforward-flags-in-the-pull-request-comment) to find out more.

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[gordonwatts]

Thanks, @BenGalewsky !!! I'll do that now! I just came here to rebase. :-)