Add `docs` action #34

Job	Run time
Lint pull request (18.x)	1m 10s
Build and test pull request (14.x, cjs, es5)	55s
Build and test pull request (14.x, cjs, es2015)	55s
Build and test pull request (14.x, esm, es5)	55s
Build and test pull request (14.x, esm, es2015)	55s
Build and test pull request (14.x, umd, es5)	51s
Build and test pull request (14.x, umd, es2015)	55s
Build and test pull request (16.x, cjs, es5)	1m 2s
Build and test pull request (16.x, cjs, es2015)	1m 1s
Build and test pull request (16.x, cjs, esnext)	1m 1s
Build and test pull request (16.x, esm, es5)	1m 0s
Build and test pull request (16.x, esm, es2015)	1m 1s
Build and test pull request (16.x, esm, esnext)	1m 0s
Build and test pull request (16.x, umd, es5)	1m 1s
Build and test pull request (16.x, umd, es2015)	1m 2s
Build and test pull request (16.x, umd, esnext)	1m 2s
Build and test pull request (18.x, cjs, es5)	57s
Build and test pull request (18.x, cjs, es2015)	1m 0s
Build and test pull request (18.x, cjs, esnext)	1m 1s
Build and test pull request (18.x, esm, es5)	55s
Build and test pull request (18.x, esm, es2015)	59s
Build and test pull request (18.x, esm, esnext)	1s
Build and test pull request (18.x, umd, es5)	1s
Build and test pull request (18.x, umd, es2015)	1s
Build and test pull request (18.x, umd, esnext)	1s
	20m 42s

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