Split the 'Server Programming' chapter into two?

Hi,

While playing around with where exactly to put the replication
origin/progress docs I once more noticed that the 'Server Programming'
book is a mix of different topics.

It currently contains:
35. Extending SQL
36. Triggers
37. Event Triggers
38. The Rule System
39. Procedural Languages
40. PL/pgSQL - SQL Procedural Language
41. PL/Tcl - Tcl Procedural Language
42. PL/Perl - Perl Procedural Language
43. PL/Python - Python Procedural Language
44. Server Programming Interface
45. Background Worker Processes
46. Logical Decoding
47. Replication Progress Tracking

To me at least 44 - 47 don't really fit well to the rest. I think we
either should invent a new category for them, or move them to
'Internals'. Maybe we could introduce 'Extending the Server' category
for those and a couple more? Candidates for that least seem to be
52. Writing A Procedural Language Handler
53. Writing A Foreign Data Wrapper
54. Writing A Custom Scan Provider
and arguably
57. GiST Indexes
58. SP-GiST Indexes
59. GIN Indexes
60. BRIN Indexes
as well.

Alternatively we could just move the above 'Internals' chapters into
'Server Programming'? But I think it's good to have split between C and
normal stuff there.

Also, shouldn't there at least be a link to
http://www.postgresql.org/docs/devel/static/xfunc-sql.html in
http://www.postgresql.org/docs/devel/static/xplang.html ?

Greetings,

Andres Freund