Fixing warnings, errors in docs build

DeepChem documentation build is having some errors and warnings ([ref](https://github.com/deepchem/deepchem/runs/7252621714?check_suite_focus=true#step:6:8)). It will be good to fix them and clean it up

Open the linked documentation build run and inspect the errors and warnings reported at step 6:8. Trace each message to the relevant documentation source or build configuration, then rerun the documentation build and confirm it completes without those errors and warnings.

Written by the indexing model from the issue text.