#28839, #28995, #29068 add/modify the file .lgtm.yml

The purpose of it should be explained in the Developer's Guide.

This depends on #30361, which adds a section for tools like this.

Depends on #33867

CC: @fchapoton @tscrim @jm58660 @tobiasdiez

Component: documentation

Issue created by migration from https://trac.sagemath.org/ticket/29520