Disable demo link and links in theme READMEs

[bep]

This relates to #423.

• People making money from Hugo themes is a great thing.
• I suspect a big motivation for Hugo theme authors that run a commercial theme site (or they sell their themes on a commercial site) is to get links to that site from gohugo.io, which has a pretty good Google ranking. This site isn't a free ad platform.
• This PR may be a bit drastic, but it should simplify some of the related discussions.

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[bep]

I'm taking the lack of respose to this as a yay.

[hugo-sid]

Thanks, @bep, for the idea (& implementation of course) of disabling the links.

One downside of this: it may cause some inconvenience for visitors who can't click on useful links.

This approach required minimal effort on our part. Another approach, though more effort-intensive, would be to periodically check the README for promotional content & de-list them.