Move PostGIS SFCGAL into it's own chapter

[robe]

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.

[Regina Obe <lr@…>]

In 64441f9/git:

Move SFCGAL into a dedicated chapter
Closes #5643 for PostGIS 3.5.0

[Regina Obe <lr@…>]

In 09a3d66/git:

Fix section abstract. References #5643 for PostGIS 3.5.0