[improve][develop-plugin] Add section for AdditionalServlet plugins

[alpreu]

Motivation

There is currently no description of adding custom servlets using Pulsar's plugin architecture

Screenshots

[tisonkun]

I'm not sure who is familiar with plugins, so cc @codelipenghui @merlimat. It's encouraged to delegate to domain experts and we know who is suitable as a reviewer then.

[alpreu]

I'm not sure who is familiar with plugins, so cc @codelipenghui @merlimat. It's encouraged to delegate to domain experts and we know who is suitable as a reviewer then.

I know @andrasbeni wrote an integration test, maybe he can help out with reviewing

[andrasbeni]

The content looks good from an engineering point of view.
I'd love to see a few words about how AdditionalServletWithPulsarService can be used instead of AdditionalServlet when the developer wants the servlet to have access to Pulsar internals (for e.g., producing messages or performing administrative tasks). But I don't insist.

[Anonymitaet]

Thanks for your contribution! Overall LGTM. I left a minor suggestion.

[alpreu]

Thanks for the review everyone! I applied your suggestions and added a small note for when to use AdditionalServletWithPulsarService

[momo-jun]

@alpreu Thanks for adding the docs!
By the way, 2.11.0 was released during the review window. Is it possible that you can also sync the contributions to pulsar-site/versioned_docs/version-2.11.x? Thank you.

[alpreu]

@momo-jun I rebased now and added the 2.11 version as well, should be good to go now :)