Pulsar Adapters: add download section (only source tarball)

[eolivelli]

This patch adds to the Download page the link to the Apache Pulsar Adapter 2.8.0 source distribution.
There is not "binary" distribution for Adapters, the artifacts (jar files) are only on Maven Central.

[eolivelli]

This is how the page will look like

[eolivelli]

In my opinion we should add to the website the documentation about how to use the various Pulsar Adapters, or at least the most important ones.
But this is a separate work

[Anonymitaet]

@eolivelli thanks for your contribution.
We have docs for adapters: https://pulsar.apache.org/docs/en/adaptors-kafka/

For the updates on the adapter chapter:

1. I plan to add Pulsar SQL doc to the adapter chapter after the Pulsar connector is contributed to Trino (WIP, code: [Connector]Add plugin for Apache Pulsar. trinodb/trino#8020, doc: [Doc] Add user guide for Pulsar connector trinodb/trino#8469)
   

2. Are there other improvements that we need to make on adapter docs?

[eolivelli]

@Anonymitaet very good

I believe that the other "adapters" module that is worth to add some docs is the Log4J2 appender
https://github.com/apache/pulsar-adapters/tree/master/pulsar-log4j2-appender

Maybe we could add a page with a minimal doc about how to start using it

[Anonymitaet]

@eolivelli thanks.
Seems that we do not have any tech inputs for Log4J2 appended, could you please provide them? Then I would like to help review, thanks.