Documentation separation ESMValTool and ESMValCore

[flicj191]

Separation of documentation of ESMValTool and ESMValCore can assist in understanding the distinct packages. It can assist in showcasing the capability of ESMValCore while maintaining good documentation for recipes in ESMValTool and a lower barrier to entry and open science.
Discussed from November Workshop: ESMValGroup/Community#192 (reply in thread)

In reviewing the documentation we have the opportunity to keep in mind some documentation principles for example: (Thanks @ehogan )

• https://diataxis.fr/
• https://canonical-example-product-documentation.readthedocs-hosted.com/en/latest/

Also discussions and suggestions on other improvements that can be addressed in separate issues which can be added here:

• Landing page improvements
  • simplified schematic and explanation of difference
  • Combined or synced landing page?
• Layout and navigation improvements
  • across the top band | Tool | Core | recipe server | .. | search
  • Chapters and navigation on the left

For separation :

• bin and remove subproject in esmvaltool readthedocs page (links will be broken)
• undo esmvalcore integration removing python file
• esmvalcore build will have readthedocs URL

Thanks for meeting @bouweandela, @ehogan, @malininae, @k-a-webb, @rbeucher

@ESMValGroup/esmvaltool-coreteam

[malininae]

At some point in the near future I volunteered to draft a simplified schematic of what is what. Once I have a first draft, I'll share here. @flicj191 thanks for opening the issue and organizing the meeting, feel free to ping me if need.