Bug 1840493 - doc/rst: fix some languages declaration r=firefox-source-docs-reviewers,webdriver-reviewers,necko-reviewers,geckoview-reviewers,devtools-reviewers,profiler-reviewers,championshuttler,whimboo,nchevobbe,julienw,amejiamarmol

Differential Revision: https://phabricator.services.mozilla.com/D196268