use application reg in docs

[andrewgsavage]

• Closes Plotting with Matplotlib from Pandas #1917
• Executed pre-commit run --all-files with no errors
• The change is fully covered by automated unit tests
• Documented in docs/ as appropriate
• Added an entry to the CHANGES file

replaces ureg = pint.UnitRegistry() with ureg = pint.get_application_registry() in the docs

[codspeed-hq]

CodSpeed Performance Report

Merging #1931 will degrade performances by 20.71%

_{Comparing andrewgsavage:app_reg (32f0bac) with master (3cc2d36)}

Summary

❌ 1 regressions
✅ 438 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	master	andrewgsavage:app_reg	Change
❌	test_create_empty_registry	2.3 ms	2.9 ms	-20.71%

[andrewgsavage]

Some small changes to the text in docs/getting/tutorial.rst
I think using pint in your projects needs a bit of a rewrite to recommend using set_application_registry

[hgrecco]

As I mentiones before, I like this change. I was discussing this with some friends which are not pint users. A lot of them ask "why get_application_registry?, Why so long?, Do you really need the application part of the name?"

Shall we bikeshed this public API? (we can still leave get_application_registry to avoid breaking things but maybe there is a better name.