Add `log4j-api` to Log4j Core 3.x binary distribution

Since the refactoring of Log4j Core 3 on top of Log4j API 2 in version `3.0.0-beta3`, the [binary distribution published on our website](https://logging.apache.org/log4j/3.x/download.html#binary-distribution) does not contain `log4j-api` any more.

To be honest, I don't entirely understand the utility of the binary distribution, but if we are going to publish one, `log4j-api` should probably be included in it.

Start with the binary distribution section of the Log4j 3.x download page and inspect how that distribution is assembled. Confirm that the published package includes log4j-api alongside Log4j Core, then verify the resulting binary distribution contents.

Written by the indexing model from the issue text.