doc/user.md: fix broken links

Thanks to #3364. See also #3349.
9 jobs for feature-a/3004-doc-markdown in 1 minute and 8 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test Tools
passed #135015
test_tools

00:00:05

 
  Test Unit
canceled #135016
test_algo_unit

00:00:58

passed #135017
test_tools_unit

00:00:04

 
  Test Functional
skipped #135018
test_algo_should
 
  Test Shouldvdj
skipped #135019
test_algo_shouldvdj
 
  Coverage
skipped #135020
algo_coverage
 
  Prepare Release
skipped #135021
allowed to fail manual
prepare_release
 
  Valgrind Unit
skipped #135022
valgrind
algo_valgrind_unit
 
  Valgrind Functional
skipped #135023
valgrind allowed to fail manual
algo_valgrind_functional