More compact and informative README

[jeanpaulrsoucy]

I've made a few commits recently aimed at making the README more compact, with the most relevant information out front, along with the making the docs directory more useful:

• Moved the section "Detailed description of data sources" to docs/data_sources/data_sources.md (e908fdc)
• Moved BibTeX citation file into docs/citation (725f3fd)
• Moved the section "Transitioning from Covid19Canada" to docs/transitioning_from_Covid19Canada (386f97a)

There are a few other changes I'd like to make:

• Turn "Available datasets" into a table with definitions for each metric, definitions for value and value_daily for each metric (Add better documentation of each value #51) and the file names for each geographic level (currently in "Included datasets" section)
• Add more information on dates that provinces stopped reporting health region-level data (currently reported without date information in the "Getting started" section
• Replacing the plots in the "Included datasets" section/the CovidTimelineCanadaPlots repository with a basic JavaScript dashboard (Create automatically updated github.io page #12)
• Document the existence of and reasons for impossible values, rather than removing these by postprocessing (Postprocessing of raw data #1)
• Add brief philosophy of dataset construction
• Add brief history and alternative names for dataset (e.g., opencovid.ca dataset)