Fix notebook in the official doc.

[ShigekiKarita]

I just noticed our gh-page doc does NOT contain notebooks https://espnet.github.io/espnet/
This is because our gh action forget submodules. Moreover, the submodule was not updated for two years.

For simplicity, we should clone the latest snapshot and generate rst during build. See https://karita.xyz/espnet/ for the updated doc

[codecov]

Codecov Report

Merging #4171 (0020349) into master (d23ff07) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4171   +/-   ##
=======================================
  Coverage   80.50%   80.50%           
=======================================
  Files         447      447           
  Lines       39063    39063           
=======================================
  Hits        31447    31447           
  Misses       7616     7616           

Flag	Coverage Δ
test_integration_espnet1	67.13% <ø> (ø)
test_integration_espnet2	50.78% <ø> (ø)
test_python	66.77% <ø> (ø)
test_utils	24.45% <ø> (ø)

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[sw005320]

Thanks a lot, @ShigekiKarita!
This is a great idea!