Adaption to TikZ picture option keys like `/tcb/tikz`

[muzimuzhi]

Starting from v6.3.0 all skins use tikzpicture as internal environment, thus the options currently documented in sec. 10.7 "TikZ Picture Option Keys", for example /tcb/tikz are now supported by all skins as well. https://github.com/T-F-S/tcolorbox/blob/f7f39a4af09b6cce028c0e8e83d6c40f56d899d1/doc/latex/tcolorbox/tcolorbox.doc.skins.tex#L2287-L2289

Accordingly their implementation can be moved from skins library to the main tcolorbox.sty file, and their documentation can be moved to the main option list, under sec. 4 "Option Keys".

\documentclass{article}
\usepackage{tcolorbox}
\tcbuselibrary{skins} % provides /tcb/tikz

\begin{document}
% "standard" skin supports /tcb/tikz now
\begin{tcolorbox}[title=Title, tikz={opacity=0.5,transparency group}]
  content
\end{tcolorbox}
\end{document}

[T-F-S]

I am not endorsing TikZ code for the standard skin, even if this works now at several places. Most prominentely, geometry nodes are not set with the standard skin and many following things expect nodes like frame or interior to be present.

So, for standard, I want to keep the focus on just using pgf code and switching to enhanced for all fancier features. Nevertheless, on the spot you marked, I will remove the two lines from the documentation, but at other places I want to still keep warnings to not use the fancier features with standard.