[DOC] Create a dropdown version list for API document

[tisonkun]

After a discussion with @momo-jun, I'm trying to add a version dropdown list in the C++ API documents so that we can always link to the latest version.

References:

• https://github.com/CGAL/cgal/blob/e1d4a96401e30cb2a3035e97c8bda44c15696cc9/Documentation/doc/Documentation/Doxyfile.in#L21
• https://docs.opencv.org/4.1.0
• https://doc.cgal.org/latest/Manual/packages.html

Originally posted by @tisonkun in apache/pulsar#18485 (comment)

[tisonkun]

I don't have time to promise working on this issue recently so any help is welcome.

[tisonkun]

An alternative is that we can add a new https://pulsar.apache.org/api/cpp/ page, which serves as a version selector. We direct new links to there and users can select the version to browser.

In this way, we can avoid hacking into Doxygen build process.

[tisonkun]

Another alternative is that we add a new placeholder said @pulsar:cpp:version@ to be resolved to the latest release version and always link to the latest version of C++ client.

As long as the following versions are backward compatible, this solution should be viable.

[momo-jun]

@BewareMyPower @Anonymitaet FYI - This issue might be relevant to the CPP release discussed on the call. It aims to avoid manual maintenance of docs in terms of the version numbers in the API doc links, since CPP client has had separate releases and version numbers.