update dqn tutorial and add envpool to docs

[gogoduan]

• I have marked all applicable categories:
  • exception-raising fix
  • algorithm implementation fix
  • documentation modification
  • new feature
• I have reformatted the code using make format (required)
• I have checked the code using make commit-checks (required)
• If applicable, I have mentioned the relevant/related issue(s)
• If applicable, I have listed every items in this Pull Request below

[codecov-commenter]

Codecov Report

Merging #526 (817a090) into master (d29188e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #526   +/-   ##
=======================================
  Coverage   94.35%   94.35%           
=======================================
  Files          63       63           
  Lines        4251     4251           
=======================================
  Hits         4011     4011           
  Misses        240      240           

Flag	Coverage Δ
unittests	94.35% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
tianshou/data/buffer/base.py	99.42% <100.00%> (ø)

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