Add `pex --docs` and several `pex3 docs` options. #2490

Job	Run time
Check GitHub Organization	1s
tox -e py312-pip24_0	3m 16s
tox -e format-check,lint-check,typecheck,vendor-check,package,docs	3m 19s
tox -e py312-pip24_0-integration	3m 22s
./dtox.sh -e py27-pip20	3m 23s
./dtox.sh -e py37-pip20	3m 20s
./dtox.sh -e py311-pip20	3m 21s
./dtox.sh -e py311-pip22_3_1	3m 23s
./dtox.sh -e py311-pip23_1_2	3m 26s
./dtox.sh -e py312-pip24_0	3m 20s
./dtox.sh -e py313-pip24_0_patched	3m 21s
./dtox.sh -e pypy310-pip20	3m 20s
./dtox.sh -e pypy310-pip22_3_1	1s
./dtox.sh -e pypy310-pip23_1_2	1s
./dtox.sh -e py27-pip20-integration	3m 22s
./dtox.sh -e py37-pip22_3_1-integration	3m 20s
./dtox.sh -e py37-pip23_1_2-integration	3m 20s
./dtox.sh -e py311-pip20-integration	3m 21s
./dtox.sh -e py311-pip22_3_1-integration	3m 24s
./dtox.sh -e py311-pip23_1_2-integration	3m 20s
./dtox.sh -e py312-pip24_0-integration	3m 18s
./dtox.sh -e py313-pip24_0_patched-integration	3m 21s
./dtox.sh -e pypy310-pip20-integration	3m 19s
./dtox.sh -e pypy310-pip22_3_1-integration	1s
./dtox.sh -e pypy310-pip23_1_2-integration	1s
Gather Final Status	1s
	1h 7m 2s

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