Add examples of configs for use cases to the documentation

[ccarouge]

Currently, it can be hard to know exactly how to modify the bench_example config file for a new test run, especially the first time someone is using benchcab. It would be useful to have examples for different use cases:

• basic
• using a local path
• with namelist patch
• with names for realisation if using the same branch at different commits
• more than 2 realisations
• running fluxsites only
• changing the PBS options
• calling me.org API

It might be possible to illustrate all these use cases with only one example of a config file with annotations instead of several. Although having several might allow users to copy/paste the case they want to start off with.