Add tools for generate C++ api doc

[tisonkun]

... and generate for 3.0.0 and 2.10.2 for testing.

You can review the first two commits and preview the result locally.

This refers to apache/pulsar#17918.
This closes apache/pulsar#17917.
This closes apache/pulsar#17498.
This closes apache/pulsar#12843.

[tisonkun]

Moving forward to do the following tasks:

• regenerate the missing API docs
• cleanup legacy scripts
• remove SNAPSHOT files under the asf-site-next branch

The solution should be better than the current status. Review comments after merge is welcome.

[michaeljmarshall]

Thanks @tisonkun! Great work!

[tisonkun]

FWW, I think generated apidocs should gradually archive to asf-site-next branch so that we keep source code tidy.

For example, I generate legacy C++ API doc and Python API doc at:

• d59ca05
• d59ca05

But I don't have a clear idea on workflow to do this archive. We can do it manually for a while and find a proper way.

Generate docs under the main branch has two advantages:

• Simple preview;
• Release manager doesn't have to understand how to commit API docs to the asf-site-next branch.

[michaeljmarshall]

That approach makes sense to me.