[WIP] Restructuring the examples section

[ArlindKadra]

What does this PR implement/fix? Explain your changes.

Restructures the examples section and adds basic examples.

How should this PR be tested?

Existing unit tests.

[codecov-io]

Codecov Report

Merging #785 into develop will decrease coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #785      +/-   ##
===========================================
- Coverage    87.74%   87.72%   -0.03%     
===========================================
  Files           36       36              
  Lines         4187     4187              
===========================================
- Hits          3674     3673       -1     
- Misses         513      514       +1

Impacted Files	Coverage Δ
openml/_api_calls.py	83.11% <0%> (-1.3%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update dcac17e...f6d4880. Read the comment docs.