DOC: add example for DataFrame.resample: keywords on and level

[DaanVanHauwermeiren]

• closes #xxxx

add examples for DataFrame specific keywords: on and level.
This can possibly fix issue #14843

See attached picture for details

[codecov-io]

Codecov Report

Merging #15627 into master will decrease coverage by -0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #15627      +/-   ##
==========================================
- Coverage   91.01%   90.99%   -0.03%     
==========================================
  Files         143      143              
  Lines       49301    49301              
==========================================
- Hits        44873    44863      -10     
- Misses       4428     4438      +10

Impacted Files	Coverage Δ
pandas/core/generic.py	96.25% <ø> (ø)	✅
pandas/io/gbq.py	25% <0%> (-58.34%)	❌
pandas/tools/merge.py	91.76% <0%> (-0.35%)	❌
pandas/core/frame.py	97.87% <0%> (-0.1%)	❌
pandas/tseries/index.py	95.39% <0%> (+0.09%)	✅

---

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 54e71a7...37233ab. Read the comment docs.

[jorisvandenbossche]

@DaanVanHauwermeiren Thanks!