Use `lowercase_with_underscore.rst` naming convention for doc files

[bobleesj]

Creating an issue for now:

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@sbillinge Do we want to use lower_with_underscore for doc .rst files? A bit hard to read diffractionobjectsexample.rst. Not sure about the convention. Did a quick search and appears NumPy is using the lower_with_underscore convention (mostly, not all) https://github.com/numpy/numpy/tree/main/doc/source/reference

@bobleesj @yucongalicechen yes, let's use this convention. But I don't worry too much for now, I will just merge this.

Originally posted by @sbillinge in #237 (comment)

[yucongalicechen]

I can take this issue! It's just changing the file names, right? @bobleesj

[bobleesj]

Yup @yucongalicechen!