[Proposal]: Add concepts guide and glossary to docs

[David Huntsperger]

Feedback welcome:

   - [Proposal] Beam concepts & glossary
   
<https://docs.google.com/document/d/1hfX48p54wQjZPghENO906yo1t64rNTKfwciMCpRm7oE/edit?usp=sharing&resourcekey=0-teElndn5JmLjY_67CuLLlQ>

Thanks!