docs: Update "Troubleshooting" to include configuration for ignoring shapes.txt

[barbeau]

Summary:

A common situation that deployers of the validator may encounter when validating large GTFS files is java.lang.OutOfMemoryError: Java heap space.

See https://github.com/etalab/transport-site/issues/2212 for an example. We hit this ourselves when processing large country-size datasets like the Netherlands.

This PR adds this scenario to the "Troubleshooting" file as well as different possible solutions.

Expected behavior:

Explain what to do when hitting java.lang.OutOfMemoryError: Java heap space in "Troubleshooting" file

Please make sure these boxes are checked before submitting your pull request - thanks!

• [x] Run the unit tests with mvn test to make sure you didn't break anything
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[barbeau]

Yes, thanks!