add: PubNub PubSub cdp

[dotmat]

Add: Code required for sending data to PubNub.

Changes

Added a CDP destination called 'PubNub' This lets you send data from PostHot to a real-time message platform for distribution.
See: http://www.pubnub.com

Checklist

• Words are spelled using American English
• PostHog product names are in title case. It's "Product Analytics" not "Product analytics". If talking about a category of product, use sentence case e.g. "There are a lot of product analytics tools, but PostHog's Product Analytics is the best"
• Titles are in sentence case
• Feature names are in sentence case. It's "Click here to create a trend insight" not "... create a Trend Insight" and so on.
• Use relative URLs for internal links
• If I moved a page, I added a redirect in vercel.json
• Remove this template if you're not going to fill it out!

Article checklist

• I've added (at least) 3-5 internal links to this new article
• I've added keywords for this page to the rank tracker in Ahrefs
• I've checked the preview build of the article
• The date on the article is today's date
• I've added this to the relevant "Tutorials and guides" docs page (if applicable)