Moving docs to tootsuite/documentation (#1550)

docs/Using-Mastodon/screenshots/compose-cw.png

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docs/Using-Mastodon/screenshots/compose-privacy.png

docs/Using-Mastodon/screenshots/content-warning.gif

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docs/Using-the-API/API.md

docs/Using-the-API/OAuth-details.md

docs/Using-the-API/Push-notifications.md

docs/Using-the-API/Streaming-API.md