Fix the sphinx config
Commit 455d780c did add the:
extensions = ["sphinx.ext.autosectionlabel"]
which provoques 2 problems:
- previous extensions (from swh.doc) are disabled,
- this autosectionlabel extension does produces a lot of "duplicate label" warnings.
So get rid of this line in conf.py and use explicit labels in
Also fix an indentation problem in this file.