environment-docs.yml: restore sphinx_rtd_theme
environment-docs.yml: restore sphinx_rtd_theme

Fix following error:

```
$ make docs                                                                                                                                     Generating docs with Sphinx ...
make[1]: Entering directory '/zstore/repos/flyingpigeon/docs'
Running Sphinx v3.0.4
Configuration error:
There is a programmable error in your configuration file:
Traceback (most recent call last):                                                                                                                      File "/home/lvu/.conda/envs/flyingpigeon-docs/lib/python3.7/site-packages/sphinx/config.py", line 319, in eval_config_file
    execfile_(filename, namespace)
  File "/home/lvu/.conda/envs/flyingpigeon-docs/lib/python3.7/site-packages/sphinx/util/pycompat.py", line 81, in execfile_
    exec(code, _globals)
  File "/zstore/repos/flyingpigeon/docs/source/conf.py", line 121, in <module>
    import sphinx_rtd_theme
ModuleNotFoundError: No module named 'sphinx_rtd_theme'

Makefile:20: recipe for target 'html' failed
```

bird-house/flyingpigeon