Added is_multilingual to @site endpoint

[sneridagh]

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📚 Documentation preview 📚: https://plonerestapi--1884.org.readthedocs.build/

[mister-roboto]

@sneridagh thanks for creating this Pull Request and helping to improve Plone!

TL;DR: Finish pushing changes, pass all other checks, then paste a comment:

@jenkins-plone-org please run jobs

To ensure that these changes do not break other parts of Plone, the Plone test suite matrix needs to pass, but it takes 30-60 min. Other CI checks are usually much faster and the Plone Jenkins resources are limited, so when done pushing changes and all other checks pass either start all Jenkins PR jobs yourself, or simply add the comment above in this PR to start all the jobs automatically.

Happy hacking!

[sneridagh]

@davisagli the builds are failing for 6.0, no clue why :( Same happens locally.

I still have to add the documentation updates.
I guess you have it already in mind, but the auto documentation generator is still pointing for using 6.0, and the Makefile command is missing for 6.1.

I added some comments, I had doubts, doing it inline.

[sneridagh]

Thanks @davisagli ! It would be great to have a release soon, so we can continue with Volto's related PR. I think @mauritsvanrees is thinking in cutting a Plone one soon.