Update POLYGLOTREADME and remove legacy versions from master

[danieltrt]

This PR has multiple deletions / updates to docs:

• Updating the README with the information that was previously only available on the documentation webpage.
• We also delete the documentation webpage since it is not maintained.
• Update the POLYGLOTREADME to remove legacy versions from the master branch
• The old versions can still be accessed here: last-version-having-legacy-piranha.
• Delete old legacy workflows