Add LibreTranslate machine translator, documentation and its test cases.

[drivard]

This PR introduces a new machine translator to the wagtail-localize project, leveraging the LibreTranslate API. The LibreTranslator class extends the BaseMachineTranslator and implements the necessary methods for translation.

It supports configuration alike

WAGTAILLOCALIZE_MACHINE_TRANSLATOR = {
    "CLASS": "wagtail_localize.machine_translators.libretranslate.LibreTranslator",
    "OPTIONS": {
        "LIBRETRANSLATE_URL": "https://libretranslate.org",
        "API_KEY": "<Your LibreTranslate api key here>",
    },
}

The LibreTranslate API can also be hosted using many methods documented on their github page https://github.com/LibreTranslate/LibreTranslate. I tested it against a self-hosted instance.

Here are some screenshots of an about us page that I translated using the LibreTranslator.

Please review and provide any feedback.

[codecov-commenter]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (a3efa8f) 92.60% compared to head (8d0d62b) 92.53%.

Files	Patch %	Lines
...ail_localize/machine_translators/libretranslate.py	76.47%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #753 +/- ## ========================================== - Coverage 92.60% 92.53% -0.07% ========================================== Files 46 47 +1 Lines 4017 4034 +17 Branches 598 600 +2 ========================================== + Hits 3720 3733 +13 - Misses 175 179 +4 Partials 122 122 ```

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[drivard]

@zerolab I have completed the changes requests. Please review.

[drivard]

@zerolab Do you have an idea of when it will be released ?

[zerolab]

Got a couple of more PRs and if all goes well, sometime next week (i.e before the end of the year)