InstallGuide: Update documentation of JDK RPM installation steps

Since jdk8 JDK and JRE RPMs become signed with OL keys installation of these RPMs on Linux distributions other than OL pops up warning that security validation of the package fails.

The reason for this warning is that the public key used for signing RPMs is not installed in the system. There is no such problem when installing RPMs on OL as this key is available by default in the system.

The problem and the evaluation is described in JDK-8246165.

We would like to update JDK/JRE RPM installation guide on Linux for jdk8 with the details on installing the public key.

The steps are the following:

If installation of JDK/JRE RPM package outputs warning to the console something like:
---
warning: jdk-8u281-ea-bin-b03-linux-amd64-20_oct_2020.rpm: Header V3 RSA/SHA256 Signature, key ID ec551f03: NOKEY
---

Then this indicates the public key used to sign this RPM needs to be installed in the system.
Key installation steps:
1. Download the key from https://yum.oracle.com/RPM-GPG-KEY-oracle-ol7:
  wget https://yum.oracle.com/RPM-GPG-KEY-oracle-ol7
2. Install the key:
  sudo rpm --import RPM-GPG-KEY-oracle-ol7

Public key installation needs to be done only once. The key installation is persistent across reboots.