Updated website based on community suggestions

[yyexela]

In response to a conversation with @mattlevine22 and @DanWaxman

Updates:

• Made pair_id to metric (E_x) more explicit in documentation on the website
• Made the mapping from training data to explicit time steps better documented on the website
• Updated documentation for def get_.*_training_timesteps in python files
• Fixed some bugs/typos
  • E4 on website was incorrectly labeled as a short-time forecast, it is a long-time forecast
  • Fixed code bugs in getting_started.rst
  • Fixed typos in contributing.rst and configuration.rst

[yyexela]

@yuezhao6371 Could you also take a look when you get the chance and let me know if you catch anything else?