documentation: rename docs.s.o/users/ (plural) to docs.s.o/user/ (singular)

The 3 pieces of docs have currently the following URLs as roots:

• docs.s.o/devel/
• docs.s.o/sysadm/
• docs.s.o/users/

2 out of 3 are singular nouns, while the third one is plural.

We should be consistent and rename the latter to /user/.

I'm marking this as high priority hoping that we fix that before it becomes popular/bookmarked.

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Migrated from T3650 (view on Phabricator)