doc: build: fix gen_compile_commands section level

The 2 subsections of this page ("Compatible IDEs" and "Usage") are using
the same header level as the title (with "===").
Because of this, they always appear in the HTML rendered ToC.

Drop the subsections one level lower by replacing "=" by "-".
This fixes the HTML rendering.

Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>