[WIP] Document the buildfarm job names used

[j-rivero]

An effort for trying to standardize the job names used in the buidlfarm. This initial draft collects the CI and install jobs for the Gazebo libraries that are currently in use, mostly generated by gazebo_libs.dsl, ignition.dsl and sdformat.dsl.

Includes changes pending in #1078.

Left some TODOS to help evolving the document or improving it.

[j-rivero]

I've refactored the some common sections and the platform in d1cf9a0. The format described is awful as an standard but should be able to describe the Jenkins jobs generated by the mentioned DSLs.

[azeey]

Thanks for working on this! The naming scheme used by the various jobs has always been a source of confusion for me. I usually end up doing a trial-and-error to find the job I'm interested in. This has especially been difficult when teaching new users or interns about our CI and which jobs they should be looking at while working on a project. Using a consistent naming scheme and having it all documented would be amazing!!