Add support for is_pattern_library context variable. Fix #156

[thibaudcolas]

Description

Addresses #156. I’ve added docs as well as a small demo – for the actual implementation & tests it was just a matter of renaming the existing variable.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have added an appropriate CHANGELOG entry

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Suggested CHANGELOG entry:

## Added

- Add a way to customise rendering of a pattern with [`is_pattern_library`](https://torchbox.github.io/django-pattern-library/reference/api/#is_pattern_library) context variable ([#156](https://github.com/torchbox/django-pattern-library/issues/156), [#167](https://github.com/torchbox/django-pattern-library/pull/167)).

[zerolab]

LGTM!