Revise documentation page, remove image

This page reviews and revises the Microsoft Teams integration page at Settings > Integrations > Microsoft Teams Notification. It contains some extras:

• Revises https://docs.gitlab.com/ee/user/project/integrations/microsoft_teams.html because it was nowhere near GitLab tone and style.
• Revises what I can from the page, because not everything is easily changed. (Not the standard formatting.)
• Attempts to revise the shared file app/helpers/services_helper.rb to trim verbosity from fields that are shared across multiple parts of the UI. This is technically out of scope, but per discussion with Craig Norris, if we can get it in here with a minimum of test-fixing, let's get this improvement in
• Revises the actual integration page (what this MR was supposed to be about!) and takes the instructions out of the UI. Puts them back in the docs where they belong.

Screenshots

header	header

Related links

• Closes #300611 (closed)
• Related to technical-writing#352 (closed)