[PostGIS] #5643: Move PostGIS SFCGAL into it's own chapter

[PostGIS]

#5643: Move PostGIS SFCGAL into it's own chapter
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 Reporter:  robe           |      Owner:  robe
     Type:  enhancement    |     Status:  assigned
 Priority:  medium         |  Milestone:  PostGIS 3.5.0
Component:  documentation  |    Version:  master
 Keywords:                 |
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 Right now PostGIS SFCGAL is a subsection of the main PostGIS reference and
 does not have it's own dedicated chapter like other extensions
 (postgis_raster, postgis_topology, postgis_tiger_geocoder).

 This is a problem for 2 reason

 1) Sometimes confuses people into thinking SFCGAL functions are available
 as part of the main postgis extension

 2) Doesn't allow postgis sfcgal functions to be easily broken out into
 logical sections like for example Raster has.

 3) Since the garden tests see it as part of postgis proper, they can't be
 tested separately.

 So the idea is to have it have it's own dedicated chapter, broken into
 sections with it's own dedicated garden tests.
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Ticket URL: <https://trac.osgeo.org/postgis/ticket/5643>
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