Iss2519 - Remove javadoc directory in zip and point users to the javadoc built by the Mkdocs static site

[jadecarino]

Why?

For galasa-dev/projectmanagement#2519

Changes

• The Mkdocs build for the Galasa static documentation site now grabs the Javadoc HTML from https://development.galasa.dev/main/maven-repo/javadoc and puts it into a directory within the site.
• The Isolated build therefore does not need to populate a javadoc directory with the same content (by building pomJavadoc.xml)
• This PR removes the pomJavadoc.xml file and any associated steps in the local and GitHub Actions build for it
• The README.txt provided in the Isolated and MVP zips now describes the location of the Javadoc as inside the galasa-docs-site directory (the Mkdocs site)

[ideeley]

Looks straightforward enough!