Add `pex --docs` and several `pex3 docs` options. (#2365) #2495

Job	Run time
Check GitHub Organization	1s
./dtox.sh -e py27-pip20	11m 54s
tox -e format-check,lint-check,typecheck,vendor-check,package,docs	6m 3s
tox -e py312-pip24_0	20m 47s
./dtox.sh -e py37-pip20	16m 41s
tox -e py312-pip24_0-integration	1h 3m 29s
./dtox.sh -e py311-pip20	16m 14s
./dtox.sh -e py311-pip22_3_1	15m 40s
./dtox.sh -e py311-pip23_1_2	15m 11s
./dtox.sh -e py312-pip24_0	15m 45s
./dtox.sh -e py313-pip24_0_patched	15m 36s
./dtox.sh -e pypy310-pip20	26m 44s
./dtox.sh -e pypy310-pip22_3_1	26m 4s
./dtox.sh -e pypy310-pip23_1_2	26m 45s
./dtox.sh -e py27-pip20-integration	22m 29s
./dtox.sh -e py37-pip22_3_1-integration	45m 44s
./dtox.sh -e py37-pip23_1_2-integration	44m 8s
./dtox.sh -e py311-pip20-integration	25m 30s
./dtox.sh -e py311-pip22_3_1-integration	29m 50s
./dtox.sh -e py311-pip23_1_2-integration	30m 36s
./dtox.sh -e py312-pip24_0-integration	35m 23s
./dtox.sh -e py313-pip24_0_patched-integration	41m 52s
./dtox.sh -e pypy310-pip20-integration	48m 54s
./dtox.sh -e pypy310-pip22_3_1-integration	59m 40s
./dtox.sh -e pypy310-pip23_1_2-integration	58m 49s
Gather Final Status	1s
	11h 59m 50s

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