Add TNPG documentation

[nicolengsy]

To close #2019 .

[codecov]

Codecov Report

Merging #2162 (93a2761) into master (3b3ad04) will decrease coverage by 0.22%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2162      +/-   ##
==========================================
- Coverage   91.51%   91.29%   -0.23%     
==========================================
  Files         197      198       +1     
  Lines       10864    10944      +80     
  Branches     1370     1374       +4     
==========================================
+ Hits         9942     9991      +49     
- Misses        666      694      +28     
- Partials      256      259       +3     

Impacted Files	Coverage Δ
src/garage/sampler/_dtypes.py	90.90% <0.00%> (-9.10%)	⬇️
src/garage/np/_functions.py	77.39% <0.00%> (-7.04%)	⬇️
src/garage/_environment.py	95.45% <0.00%> (-1.99%)	⬇️
src/garage/sampler/multiprocessing_sampler.py	89.61% <0.00%> (-1.30%)	⬇️
src/garage/_dtypes.py	96.01% <0.00%> (-1.19%)	⬇️
src/garage/sampler/vec_worker.py	88.98% <0.00%> (-0.74%)	⬇️
src/garage/trainer.py	90.94% <0.00%> (-0.18%)	⬇️
src/garage/torch/_functions.py	94.68% <0.00%> (-0.06%)	⬇️
src/garage/tf/algos/te.py	100.00% <0.00%> (ø)
src/garage/tf/algos/npo.py	96.46% <0.00%> (ø)
... and 22 more

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[nicolengsy]

@mergify rebase

[mergify]

Command rebase: failure

Branch rebase failed
Git reported the following error:

First, rewinding head to replay your work on top of it...
Applying: Init TNPG doc
Using index info to reconstruct a base tree...
M	docs/index.md
Falling back to patching base and 3-way merge...
Auto-merging docs/index.md
CONFLICT (content): Merge conflict in docs/index.md
error: Failed to merge in the changes.
Patch failed at 0001 Init TNPG doc
Use 'git am --show-current-patch' to see the failed patch

Resolve all conflicts manually, mark them as resolved with
"git add/rm <conflicted_files>", then run "git rebase --continue".
You can instead skip this commit: run "git rebase --skip".
To abort and get back to the state before "git rebase", run "git rebase --abort".

err-code: AFD23

Hey, I reacted but my real name is @Mergifyio