Rework documentation

After discussion with @phackstock, I have compiled some information on ece-docs and other ScSe documentation.

## `ece-docs`
Topic | Requirements
-|-
IAMC timeseries data format | -
Scenario DBs: generations, access/permission, registration, versioning, processing, API | Update w/ group managers, scse-files project template
Naming conventions: variables, regions, country names, NUTS | -
Open-source packages: pyam, nomenclature, ixmp4, pysquirrel | -
FAQ | Update questions
Tutorial: querying IIASA DBs | -
Tutorial: local scenario processing | Add other relevant nomenclature CLI commands  
Requirements for processing modules | -

## `ixmp-server-docs`
- Tutorials: introduction to the Scenario Explorer, user registration, making a figure, documentation, download, uploading, log
- ixmp REST API
- Requirements for processing modules (duplicate)

This content should not be removed due to funding acknowledgements potentially still being enforced.
They can be moved to a section in `ece-docs`, some of it in an Archive session.

# Priorities
One initial intention for **`ece-docs`** was to be a compilation of information for the ScSe infrastructure and software stack.

- Develop guides/video tutorials for new generation of Scenario Explorers
- Make additions to documentation for missing elements
- Structure sections focused on each part of the infrastructure: Manager, Files, Processing, Workspaces, etc.
- Develop user stories to facilitate use of the documentation (e.g.: project manager, project-internal user, modeler, etc.)
- Each user story reflects a potential/archetypical user of the ScSe infrastructure
- Each user story will have documentation pages and tutorial steps that are more/less relevant to them
- An initial page should help end users identify their stories (e.g.: "If you are a project manager, see steps 3, 4, and 6")
- Reorganise where necessary for improved navigation