pgsql: Doc: move info for btree opclass implementors into main document

Doc: move info for btree opclass implementors into main documentation.

Up to now, useful info for writing a new btree opclass has been buried
in the backend's nbtree/README file. Let's move it into the SGML docs,
in preparation for extending it with info about "in_range" functions
in the upcoming window RANGE patch.

To do this, I chose to create a new chapter for btree indexes in Part VII
(Internals), parallel to the chapters that exist for the newer index AMs.
This is a pretty short chapter as-is. At some point somebody might care
to flesh it out with more detail about btree internals, but that is
beyond the scope of my ambition for today.

Discussion: https://postgr.es/m/23141.1517874668@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3785f7eee3d98bc0a7ac2576aac9c0be974217d4

Modified Files
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doc/src/sgml/btree.sgml | 267 +++++++++++++++++++++++++++++++++++++++
doc/src/sgml/filelist.sgml | 1 +
doc/src/sgml/postgres.sgml | 1 +
doc/src/sgml/xindex.sgml | 15 +--
src/backend/access/nbtree/README | 53 --------
5 files changed, 276 insertions(+), 61 deletions(-)