add example to sqlite docs illustrating custom compiler ext. with Bigint + primary key

[Mike Bayer]

tried to get a trigger to do this on sqlite, it can be done, but again not in a way that is backend neutral (the pk col has to be nullable, or have a server default set on it). so lets illustrate the example shown in #2074.

[Mike Bayer]

• document fully how to use autoincrement w/ SQLite including non-Integer column types, fixes #2075

→ <<cset c581b907b3f1>>

[Mike Bayer]

• document fully how to use autoincrement w/ SQLite including non-Integer column types, fixes #2075

→ <<cset 0771e165063c>>