Add documentation for command-line tools

Test Plan

make -C docs html adds a new file

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Migrated from D3028 (view on Phabricator)

Build is green

Patch application report for D3028 (id=10757)

Rebasing onto b7ec05db...

Current branch diff-target is up to date.

Changes applied before test

commit dca9c5fc666d5f7039feda1de0357b23938af340
Author: Nicolas Dandrimont <nicolas@dandrimont.eu>
Date:   Fri Apr 17 17:14:38 2020 +0200

    Add documentation for command-line tools

See https://jenkins.softwareheritage.org/job/DCORE/job/tests-on-diff/4/ for more details.

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