Add how to obtain the unicode of the symbols in docstrings

[lbenet]

No description provided.

[OlivierHnt]

Thanks @lbenet for the PR. When one types ? ⊏ (or any other unicode symbol), Julia already informs the user on how to write it down. E.g.

help?> ∈
"∈" can be typed by \in<tab>
[.....]

During our last call, I thought that the purpose of this PR was to add to the documentation something about the submodule IntervalArithmetic.Symbols (which is almost undocumented except for https://juliaintervals.github.io/IntervalArithmetic.jl/stable/manual/construction/#More-constructors).
E.g. we should add a markdown table like

| Symbol             | Equivalent Function         | Description                                                  |
|-------------------|----------------------------|--------------------------------------------------------------|
| `..(a, b)`         | `interval(a, b; format=:infsup)` | Create interval `[a, b]` using bounds                        |
| `±(m, r)` (`\pm<tab>`)      | `interval(m, r; format=:midpoint)` | Create interval `[m - r, m + r]` using midpoint-radius form  |
| `≛(x, y)` (`\starequal<tab>`)    | `isequal_interval(x, y)`   | Check interval equality                                      |
| `⊑(x, y)` (`\sqsubseteq<tab>`) | `issubset_interval(x, y)`  | Check if `x` is a (non-strict) subset of `y`                |
| `⋤(x, y)` (`\sqsubsetneq<tab>`) | `isstrictsubset(x, y)`     | Check if `x` is a strict subset of `y`                      |
| `⪽(x, y)` (`\subsetdot<tab>) | `isinterior(x, y)`         | Check if `x` is in the interior of `y`                      |
| `⪯(x, y)`  (`\preceq<tab>`)        | `precedes(x, y)`           | Precedes relation                         |
| `≺(x, y)` (`\prec<tab>`)  | `strictprecedes(x, y)`     | Strictly precedes relation                                   |
| `⊔(x, y)` (`\sqcup<tab>`)        | `hull(x, y)`               | Interval hull of `x` and `y`                                 |
| `⊓(x, y)` (`\sqcap<tab>`)       | `intersect_interval(x, y)` | Intersection of intervals                                    |
| `∅`  (`\emptyset<tab>`)         | `emptyinterval()`          | Empty interval                                           |
| `ℝ`   (`\bbR<tab>`)             | `entireinterval()`         | Entire real line                                         |

[lbenet]

Uuups 😮 :blush... I must admit I didn't remember the trick with the UTF acceted symbols. I'll change this PR following the suggestion to add a table with the symbols.

[OlivierHnt]

@lbenet if you have a bit of time, it would be nice to merge this PR over the weekend, and release what should be the last pre-1.0 release of IntervalArithmetic.

[lbenet]

@OlivierHnt I've removed the trivial suggestions of the former commits and pushed the suggested changes., whichI inserted the docs in the manual section. I will not have time this afternoon nor tomorrow, so feel free to make any change you think is worth in this PR, so it does not slow down the release of pre-1.0.

[lbenet]

Thanks a lot!