Plugin discovery documentation contained information about installing
Elasticsearch 2.0 and installing an oracle JDK, both of which is no
longer valid.
While noticing that the instructions used cleartext HTTP to install
packages, this commit replaces HTTPs links instead of HTTP where possible.
In addition a few community links have been removed, as they do not seem
to exist anymore.
Co-authored-by: Alexander Reelsen <alexander@reelsen.net>
5a2c6f0d4f