Additional Chapter for Tutorial

Our documentation explains many details about commands, tools, 
parameters in detail and with high accuracy. Nevertheless my impression 
is that we neglect the 'big picture': why certain processes exist and 
what their relation to each other is, summary of strategies, 
visualization of key situations, ... . People with mature knowledge 
don't miss this information because they know all about it. But for 
beginners such explanations would be a great help. In the time before 
GSoD 2019 we had similar discussions.

I plan to extend over time the part 'Tutorial' by an additional chapter 
with an overview about key design decisions and basic features. The 
typical audience should consist of persons with limited pre-knowledge in 
database systems and some interest in PostgreSQL. In the attachment you 
find a patch for the first sub-chapter. Subsequent sub-chapters should 
be: MVCC, transactions, VACUUM, backup, replication, ... - mostly with 
the focus on the PostgreSQL implementation and not on generic topics 
like b-trees.

There is a predecessor of this patch: 
https://www.postgresql.org/message-id/974e09b8-edf5-f38f-2fb5-a5875782ffc9%40purtz.de 
. In the meanwhile its glossary-part is separated and commited. The new 
patch contains two elements: textual descriptions and 4 figures. My 
opinion concerning figures is set out in detail in the previous patch.

Kind regards, Jürgen Purtz