glossary.rst: Initial commit

Contains definitions of terms commonly used in the ZeroVM ecosystem.
index.rst: reword the heading of the main TOC

I want to add a `Glossary` to the TOC, and so changing "Getting
Involved" to a more general "Getting Started with ZeroVM" for other
types of general information.

For example, in the future I also want to add tutorials, installation
guides, etc., and such topic make more sense under the new heading.
index.rst: add glossary to main TOC
index.rst: remove trailing whitespace
Merge pull request #10 from larsbutler/glossary

Glossary
tox: remove unnecessary basepython setting

Tox would already use "python" as the default here.
glossary: use proper glossary directive
glossary: fix internal term references

Some terms were shortened to make it easier to reference them.
glossary: rewrap text
glossary: fix casing
glossary: automatically sort entries
glossary: pull out expanded abbreviations

This way you can use both "NaCl" and "Native Client" as terms in the
rest of the documentation.
glossary: add "ZeroVM Shell" for "zvsh"
Merge pull request #11 from mgeisler/glossary-directive

Use sphinx glossary directive for glossary page