gh-38798: adding method tikz to class Graph
Currently, we can create a TikzPicture from a graph as follows, but is
raises an experimental feature warning:
```
sage: from sage.misc.latex_standalone import TikzPicture
sage: g = graphs.PetersenGraph()
sage: t = TikzPicture.from_graph(g)
<ipython-input-20-c4b6306d5e76>:1: FutureWarning: This
class/method/function is marked as experimental. It, its functionality
or its interface might change without a formal deprecation.
See https://github.com/sagemath/sage/issues/20343 for details.
t = TikzPicture.from_graph(g)
sage: t
```
It was declared experimental during the review of #20343 because it
should rather be a method of the class Graph.
This is what we do in this PR: we add a tikz method to the class Graph.
### :memo: Checklist
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- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
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- [x] I have created tests covering the changes.
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- [ ] and checked the documentation preview.
### :hourglass: Dependencies
None
### Screenshots
As a consequence, the following now works in Jupyter. By default, it
uses the dot2tex format if dot2tex is available:
### Works well with sagetex
For example:
```
\begin{sagesilent}
g = graphs.PetersenGraph()
tikz = g.tikz()
\end{sagesilent}
\begin{center}
\sageplot[scale=.5][pdf]{tikz}
\end{center}
```
URL: https://github.com/sagemath/sage/pull/38798
Reported by: Sébastien Labbé
Reviewer(s): David Coudert, Frédéric Chapoton, Sébastien Labbé, Xavier Caruso
