Update: Sending and Receiving Messages on Thunderbird

[rtanglao]

• KB article link: Sending and Receiving Messages on Thunderbird
• [x] approve last edit from Mozinet, 263263 and if need be do another edit to fix typos in that rev
• [x] fix broken link for "Configure an account". It should be: https://support.mozilla.org/kb/manual-account-configuration <-- probably best to change to [[]] style link instead of a direct link
• [x] for macOS, Preferences is now Settings just like Windows and Linux
• [x] Tools->Account Settings is gone on macOS, replaced by Click {button ≡} > {menu Account Settings} or Thunderbird menu > Account Settings, Tools -> Account Settings is still there on Windows but not Linux, to make it less confusing just document Click {button ≡} > {menu Account Settings}
• [x] File > Get New Messages for is no longer accessible from the app menu i.e. ≡
• [x] change buttons for writing a message and getting new messages to the new TB115 buttons
• [x] App Menu > Create > New
• [x] if somebody has time, update screenshots :-)

[rtanglao]

i asked @wsmwk and @dannycolin or other person with privs to review in #tb-support-crew and on slack so closing this issue