docs: add "How to manage 12-factor app charms" #221

Job	Run time
lint / files	48s
test / Fast tests (jammy, amd64)	5m 11s
test / Fast tests (noble, amd64)	4m 48s
test / Minimum dependencies (all tests)	6m 2s
test / Fast tests (macos-13)	8m 14s
test / Slow tests (noble, 3.10)	6m 42s
test / Fast tests (macos-14-large)	6m 49s
test / Slow tests (macos-14-large, 3.10)	3m 14s
test / Fast tests (windows-2019)	10m 1s
test / Fast tests (windows-2022)	9m 31s
	1h 1m 20s

Provide feedback