improve cp documentation with some illustration examples

[ndeloof]

closes#3431

- What I did
Added examples on docker cp reference documentation

[codecov-commenter]

Codecov Report

Merging #3463 (cbaca6b) into master (ab7cc48) will not change coverage.
The diff coverage is n/a.

@@ Coverage Diff @@## master #3463 +/- ##
=======================================
Coverage 58.99% 58.99% =======================================
Files 284 284 Lines 23848 23848 =======================================
Hits 14068 14068 Misses 8920 8920 Partials 860 860 

[thaJeztah]

Thanks! Left one comment to be fixed (and some nits while at it 😅)

[thaJeztah]

Can you change this to a H3? The yamldocs we generate for docs.docker.com collect everything under the H2 Description and H2 Examples sections. Putting this as a H2 causes it to be skipped (but it will be included if it's a H3 within the "examples" section

Suggested change

	## Corner cases
	###Corner cases

[thaJeztah]

While you're fixing the H2 -> H3, could you also put a blank line between the text and the "code" block? It's not strictly needed (we did have issues with that in the past, but I think those are solved), but it's consistent with how we use them everywhere

So;

Some text before the example code
```console
$ echo "I am the example code"
```

[thaJeztah]

Same here (and the next one below)

[thaJeztah]

LGTM, thanks!