Enhance Cube slicing docs

[hsteptoe]

🚀 Pull Request

Description

Some relatively minor enhancements to the docs for the cube.slices() and cube.slices_over().

• Made some of the wording clearer
• Modified examples to use iris-sample-data
• Added example of using dimension indices for slicing.

---

Consult Iris pull request check list

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.77%. Comparing base (7c313ff) to head (13abd1a). Report is 12 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5735   +/-   ##
=======================================
  Coverage   89.76%   89.77%           
=======================================
  Files          93       93           
  Lines       22982    22995   +13     
  Branches     5006     5011    +5     
=======================================
+ Hits        20630    20643   +13     
  Misses       1622     1622           
  Partials      730      730           

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[hsteptoe]

N.B. I've aimed for judicious use of # doctest: SKIP directive to deal with cases where example output would be too long to be useful, or where I seem to get stuck in a linter-modified-test-fail loop with cube printing

[CLAassistant]

All committers have signed the CLA.

[trexfeathers]

Brilliant, thanks again!