docs: change `troubles` references to `instability`
Per https://www.mercurial-scm.org/wiki/CEDVocabulary. There are a couple
instances of 'troublesome' which probably need a word, as deriviatives of
'instability' didn't really fit. At least some of these are in the unlinked
obs-terms.rst page.
syntax: re
/figures/[^/]+\.png$
^html/
\.pyc$
~$
\.swp$
\.orig$
\.rej$
^tests/.*\.err$
^tests/easy_run.sh$
^build/
^dist/
^MANIFEST$
^docs/tutorials/.*\.rst$
\.ico$
tests/\.testtimes
^docs/training/graphs/
^docs/training/html/
^docs/training/index.html
^docs/training/graphviz-images/
^docs/training/img/
^docs/training/output/
^docs/training/pandocfilters/