docs(io): add modflow extended description to IO guide #3586

Job	Run time
Build	8m 3s
Check format	1m 50s
Smoke test	13m 38s
Extended testing (ubuntu-22.04)	27m 27s
Test Intel fortran (ubuntu-22.04)	41m 38s
Extended testing (macos-13)	1h 0m 56s
Test GNU fortran (macos-13, false)	41m 36s
Test Intel fortran (macos-13)	34m 56s
Extended testing (windows-2022)	53m 4s
Test Intel fortran (windows-2022)	35m 21s
Test GNU fortran (macos-14, false)	23m 36s
Test GNU fortran (macos-14, true)	35m 13s
Test GNU fortran (ubuntu-22.04, false)	48m 34s
Test GNU fortran (ubuntu-22.04, true)	1h 43m 40s
Test GNU fortran (windows-2022, false)	38m 39s
	9h 28m 11s

Provide feedback