Add a link to access uptime specific documentation

[bhavyarm]

Kibana version: 7.2.0 BC

Original install method (e.g. download page, yum, from source, etc.): from staging

Describe the bug: Uptime app doesn't have the link to access uptime specific documentation.

Other apps do and its a handy thing to have.

Uptime:

Example Canvas/code/maps:

[elasticmachine]

Pinging @elastic/uptime

[justinkambic]

Thanks for this @bhavyarm - we'll shoot to get this completed for 7.2.1/7.3.0.