[rfile] add basic rfile tutorials

[silverweed]

This Pull request:

Adds basic tutorials for RFile both in C++ and python.

Checklist:

• tested changes locally
• updated the docs (if necessary)

[hahnjo]

You will need to veto the tutorial if root7 is disabled, I guess

[silverweed]

You will need to veto the tutorial if root7 is disabled, I guess

In the end we decided to build RFile regardless of that flag, so the test can be run even without (assuming that's what we want)

[vepadulano]

Nice! See minor comments

[hageboeck]

Last thing from my side:
Maybe we could make the tutorial findable from here

This would entail putting it in the group tutorial_io, and maybe updating https://github.com/root-project/root/blob/master/tutorials/io/index.md

[github-actions]

Test Results

    22 files      22 suites   4d 0h 25m 57s ⏱️
 3 743 tests  3 743 ✅ 0 💤 0 ❌
80 395 runs  80 395 ✅ 0 💤 0 ❌

Results for commit 681bf71.

♻️ This comment has been updated with latest results.