feat(doc): add waiting queue documentation

[ltorje-8x8]

Documentation on how to implement a waiting queue backend and examples on how to use it.

[jitsi-jenkins]

Hi, thanks for your contribution!
If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.

[damencho]

Maybe create a sub folder in extra large conferences, name waiting-queue and move all images and files there.

[saghul]

I'd say this PR should really go into the handbook, we do have a section on cloud API: https://jitsi.github.io/handbook/docs/devops-guide/cloud-api/

As a further improvement, we could consider using this plugin to make it interactive: https://github.com/PaloAltoNetworks/docusaurus-openapi-docs