Generating labels could prepend filename (internally) to create globally unique labels #36

Describe the problem

When adding in simple examples such as:

```{exercise}
:label: exercise-1
```

this will cause a warning to be issued if another exercise is labeled the same way in another document.

This is a pretty common pattern and the context should be handeled by the extension.

The downside is referencing would need to be documented if you want to reference an example from another document such as using <filename>:example-1 as a standard reference

Link to your repository or website

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Steps to reproduce

1. build a document with the same label in two documents
2. build and a warning will be issued such as

/Users/matthewmckay/repos-collab/continuous_time_mcs/ctmc_lectures/uc_mc_semigroups.md: WARNING: duplicate example label 'example-1', other instance in /Users/matthewmckay/repos-collab/continuous_time_mcs/ctmc_lectures/generators.md

The version of Python you're using

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Your operating system

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Versions of your packages

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Additional context

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