Docs - README.md community channels removal + link

[petermarshallio]

I spotted that the Slack registration links here didn't have the ?v=1 trick - but thought it might just be better to propose a leaner approach: link directly to the community page itself (which contains the necessary links to lots of stuff) instead of the separate heading here.

• Removed explicit links to project channels in favour of a link direct to the Community page on druid.apache.org.
• Updated nav to match remaining headings in the README.

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This PR has:

• [X] been self-reviewed.

[petermarshallio]

@paul-rogers I did some faffing. How's this look... @2bethere thought you might have have a valuable opinion, too

[paul-rogers]

@petermarshallio, thanks for the improvements!

LGTM (non-binding)