WIP: What are manifolds?

[mateuszbaran]

A new general descriptions of what manifolds are.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.97%. Comparing base (76d6a5f) to head (87c50f2).

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[kellertuer]

I carefully read this. It I a very good idea to introduce this.

1. I would like to rephrase the beginning a bit, since currently it starts what we can not do. However, for now all we need and do implement is the definition used in the next section. So there is no need for the negativity.
2. one could mention a bit, that some links point even to Manifolds.jl since that is not yet part of the interface – all links with icons upfront like at_point
3. Also in topological part we again have a bit of a negative phrasing, but my larger problem is that the points 1-4 are nice to know but might yield the impression we would cover those already. We do not.
4. The same as 3) for the Finsler manifolds. Currently these are not supported neither here in ManifoldsBase nor in Manifolds.jl – so that might be misleading in a tutorial?
5. Finally, since the tutorial does not involve any code (or is that planned) it could also directly be written in markdown within the docs as a normal docs page – would that be reasonable?

I can try the next few days to write an introduction, that is positive, maybe not covering everything, but covering everything that is needed to get started and generalise or specialise it later when necessary. For a starl the topological charts are enough and there is no need to know that other definitions need variants of this.