Add 2.5 Documentation to the website

Following the example from HBASE-24487, will create PR for both the main repo and the hbase-site repo.

Generating the site documentation from main repo, copy to site, as follows:

hbase $ git checkout rel/2.5.0
hbase $ mvn clean site site:stage -DskipTests
hbase-site $ git checkout asf-site
hbase-site $ cp -r ../hbase/target/staging ./2.5
hbase-site $ git add 2.5

Then, update the site structure to include a reference to the new content, following https://github.com/apache/hbase/pull/2060