Fix sphinx documentation build

The four commits in that diff fix the swh development documentation build (no more sphinx warnings).

Below is the commit log:

commit 316d17ba2d5d90a7bac30a13dd2912fcbd5d4e81 (HEAD -> fix-sphinx-dev-doc-build)
Author: Antoine Lambert <antoine.lambert@inria.fr>
Date:   Mon Jan 20 11:14:44 2020 +0100

    docs: Fix sphinx warnings
    
    Notably remove references to old swh loader modules and add docs
    for swh-search and swh-icinga-plugins.
    
    Related to #2188

commit a6affb78465697c69dbf6ecb5945e08578adf37f
Author: Antoine Lambert <antoine.lambert@inria.fr>
Date:   Mon Jan 20 11:12:15 2020 +0100

    sphinx/conf.py: Fix path to swh logo
    
    Related to #2188

commit f425c20e4011fb776e1d39637b1c70259f4f2cbd
Author: Antoine Lambert <antoine.lambert@inria.fr>
Date:   Mon Jan 20 11:10:34 2020 +0100

    docs/Makefile: Exclude some modules from apidoc
    
    Django settings modules and swh-dataset does not contain any relevant API
    documentation and apidoc generates errors and warnings while processing
    them.
    
    Related to #2188

commit d7bb5b4253e2185949596e5c8536e32daa143d2b
Author: Antoine Lambert <antoine.lambert@inria.fr>
Date:   Mon Jan 20 11:20:51 2020 +0100

    requirements-swh-dev.txt: Update and reorder dependencies
    
    Add swh-search and swh-icinga-plugins.
    
    Dependencies need to be ordered in a way that ensure only development versions
    will be used (not the release ones hosted on PyPI).
    
    Related to #2188

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Migrated from D2564 (view on Phabricator)