Added PostgreSQL internals learning materials in Developer FAQ

Hello,

FYI
I collected Web resources to learn PostgreSQL internals in the Developer FAQ page.  I did this because I need to
provideself-education materials for new developers.  Specifically, I modified "How is the source code organized?", and
added"What information is available to learn PostgreSQL internals?".
 

Developer FAQ
https://wiki.postgresql.org/wiki/Developer_FAQ#Development_Tools_and_Help


I borrowed most of the information from the following discussion.  I'd appreciate it if you could other useful
information.

On How To Shorten the Steep Learning Curve Towards PG Hacking...

https://www.postgresql.org/message-id/flat/29ce9fa2-63d1-001b-74f4-a26e23500acb%40lab.ntt.co.jp#29ce9fa2-63d1-001b-74f4-a26e23500acb@lab.ntt.co.jp


Regards
Takayuki Tsunakawa