pgsql: doc: Replace list of drivers and PLs with wiki link

doc: Replace list of drivers and PLs with wiki link

The list of external language drivers and procedural languages was
never complete or exhaustive, and rather than attempting to manage
it the content has migrated to the wiki. This replaces the tables
altogether with links to the wiki as we regularly get requests for
adding various projects, which we reject without any clear policy
for why or how the content should be managed.

The threads linked to below are the most recent discussions about
this, the archives contain many more.

Backpatch to all supported branches since the list on the wiki
applies to all branches.

Author: Jonathan Katz <jkatz(at)postgresql(dot)org>
Discussion: https://postgr.es/m/169165415312.635.10247434927885764880@wrigleys.postgresql.org
Discussion: https://postgr.es/m/169177958824.635.11087800083040275266@wrigleys.postgresql.org
Backpatch-through: v11

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/ee99330a0b39c47a7ac44c6c8fdd74e3da841ba5

Modified Files
--------------
doc/src/sgml/external-projects.sgml | 158 ++++--------------------------------
1 file changed, 18 insertions(+), 140 deletions(-)