Remove unrelated CMS compare & concepts content from this repo

[mattheworiordan]

This docs repository has a bunch of content that does not belong here, it was introduced for legacy reasons.

Compare content

See https://github.com/ably/docs/tree/master/content/compare

Internal issue to remove + all functionality - https://ably.atlassian.net/browse/WEB-273

Topic and marketing content

See https://github.com/ably/docs/tree/master/content/design-patterns, https://github.com/ably/docs/tree/master/content/concepts, and possibly https://github.com/ably/docs/tree/master/content/core-features (although this needs to be reviewed by @MarkWoulfeAbly first.

Internal issue to remove this content - https://ably.atlassian.net/browse/FEA-465

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