An initial rework of the "Concepts" documentation

[andrewgodwin]

This is the first part of me trying to improve Airflow's documentation to make it more consistent and understandable.

This takes the old, single-page Concepts document and turns it into a new Concepts section, rewrites some of the text to have consistent terminology (upstream/downstream, Task vs Operator, etc.) and tries to make it generally more accessible and high-level. It also moves a couple of other pages in that directly fit with the existing content (Scheduler and Smart Sensors).

Once this is in, I would then like to take on a high-level reorganisation of the other documents, grouping them a bit better (installation/deployment, tutorials, howto) and also making a better overall concept split between "reference" documentation (exhausive, technical details and important notes - not autogenerated API docs) and "concept" documentation (high-level design explanation).

A considerable amount of what is currently in "Concepts" should actually be in a "Reference" section, in my view, but I've kept the current grouping for now to make this PR even slightly approachable.

[mik-laj]

I am not sure if this page is well split. We on this page now have mixed up information on high-level architecture, as well as code snippets. Can you check it?

[andrewgodwin]

There's only two code snippets in there, mostly to illustrate the dependency declarations - what else would you be looking for?

[uranusjr]

I did not read the concepts documents, but here are some very minor suggestions

[github-actions]

The PR is likely OK to be merged with just subset of tests for default Python and Database versions without running the full matrix of tests, because it does not modify the core of Airflow. If the committers decide that the full tests matrix is needed, they will add the label 'full tests needed'. Then you should rebase to the latest master or amend the last commit of the PR, and push it with --force-with-lease.