[DOC] Add instruction for ssh key generation

[DOC] Add instruction for ssh key generation

OpenSSH 8.8+ disables the support for RSA/SHA1. For developers in this scenario uses "ssh-keygen -t rsa" command, the ssh key can be generated successfully. But it will come back" Permission Denied (Public Key)" when they try to pull/push code through ssh method. Few solutions in google will tell it could be the OpenSSH reason which may cause some time for trouble shooting. So instruction is added here.

Change-Id: Iaababa9e25e87604b1bc6464f65156ec6e6b2d31
Signed-off-by: qingshuting <qingshuting1@huawei.com>
Issue-ID: DOC-790