Update docs.rst with punctuation corrections (#4010)
Update docs.rst with punctuation corrections (#4010)

* Update docs.rst with punctuation corrections

This PR updates `docs.rst` (docs landing page) with some corrections and improvements (mostly punctuation).

One change I'm not very certain about is the lower casing of **Pure Python** to **pure Python**. I googled about the former style but I only found references in the **pure Python** format. I'm addressing this issue because in the documentation there are both **Pure Python** and **pure Python** occurrences so I decided on the one I've found most used online.

* Revert removal of serial commas
dask/dask,mrocklin/dask,dask/dask,jakirkham/dask,jcrist/dask,blaze/dask,ContinuumIO/dask,jakirkham/dask,ContinuumIO/dask,jcrist/dask,blaze/dask,mrocklin/dask