Announce Telegram, Slack, and give general updates

[sullivanmatt]

A small post announcing the new Telegram support as well as the Slack workspace. Added a new category and changed the notification code so that only Twitter will ever consume the news feed.

For Site Improvements

• [x] Check "Allow edit from maintainers" option in pull request so that additional changes can be pushed by the Bug Alert team if needed.
• [x] I have performed a self-review of my own code.
• [ ] I have commented my code, particularly in hard-to-understand areas.
• [x] My changes generate no new warnings.
• [x] I have run Pelican locally to ensure my changes have not disrupted the look, feel, or functionality of the site.
• [x] I have checked my code and corrected any misspellings.
• [x] I have added a pull request description which details the nature of this change.

@mattlorimor