[tutorials] Get rid of experimental category

[mdessole]

This Pull request:

• moves RCanvas tutorials to visualisation
• deletes RHist tutorials
• moves RBrowser and RFileDialog tutorials to visualization/webgui and removes warnings (not in experimental namespace anymore)
• moves RFitPanel tutorial to visualization/webgui

[linev]

RCanvas is still experimental.
Also RFitPanel is remains in Experimental namespace.

Therefore related tutorials can remain in the experimental section

[mdessole]

RCanvas is still experimental. Also RFitPanel is remains in Experimental namespace.

Therefore related tutorials can remain in the experimental section

Actually, the experimental category was created during the ROOT hackathon last November, see #17097. During the RRetreat, we agreed on getting rid of this category. Tutorials showcasing experimental features should be placed in the relevant category, but they should be labelled with a warning message.

[linev]

But fitpanel.cxx can be kept?
It shows usage of RFitPanel with RHist classes.

[github-actions]

Test Results

    17 files      17 suites   3d 21h 43m 59s ⏱️
 2 730 tests  2 730 ✅ 0 💤 0 ❌
45 035 runs  45 035 ✅ 0 💤 0 ❌

Results for commit 4f0a24b.

♻️ This comment has been updated with latest results.

[linev]

@mdessole Can you recover fitpanel.cxx and rebase PR?

[linev]

While RHist class will be removed,
one of course should remove related code in RFitPanel and and RBrowser

[mdessole]

Debian125 failure not related to code changes, merging.