Update SUMO pages and MDN docs to reflect keyboard shortcut

[punamdahiya]

With #5089 fix, user should be able to use keyboard to take shortcuts and needs related documentation on SUMO updated.

Eventually we should consolidate dev tools screenshots documentation on mdn and screenshots extension SUMO help pages so that users can easily differentiate between two.

https://developer.mozilla.org/en-US/docs/Tools/Taking_screenshots https://support.mozilla.org/en-US/kb/firefox-screenshots https://support.mozilla.org/en-US/kb/how-to-make-screenshots https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem See https://github.com/mozilla-services/screenshots/pull/5229#issuecomment-452396971