doc: && not googleable by "postgres select points by extent"

[komzpa]

User struggled writing a point-in-polygon query. Initial google search was "postgres select points by extent". It brought user to ST_Extent, and later ST_Within.

Instead:

• it should have brought user to && which should say word "extent" more
• ST_Extent should have example to &&
• && should recommend checking if ST_Intersects is fine too

[pramsey]

Add cross-reference from && to ST_Intersects and vice versa

[Regina Obe <lr@…>]

In 63a3c14/git:

Add cross reference for &&, ST_Intersects, ST_Extent to each other. Closes #4593 for PostGIS 3.1.0