Reorganize, expand, improve internal documentation

Reorganize existing READMEs and other documentation files into mdbook
format. The resulting Table of Contents is a mix of placeholders for
docs that we should write, and documentation files that we already had,
dropped into the most appropriate place.

Update the Pageserver overview diagram. Add sections on thread
management and WAL redo processes.

Add all the RFCs to the mdbook Table of Content too.

Per github issue #1979

docs/pageserver-processing-wal.md

@@ -0,0 +1,5 @@
+# Processing WAL
+
+TODO:
+- diagram that shows how incoming WAL is processed
+- explain durability, what is fsync'd when, disk_consistent_lsn