Add prominent Extensions link in Download page

[AtesComp]

In the Download page, add an Extensions section below the 'Other platforms and versions' to make the Extensions more visible to users.

Also, update the RDF Transform extension text in the Extensions page.

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[wetneb]

See the preview at https://deploy-preview-351--openrefine-website.netlify.app/download, towards the bottom.

[thadguidry]

Looks good!

Side Note: Gosh I just noticed that those blue links are so close to black and almost make the links invisible depending on how large the font sizing is across the site. Hmm... I think we might make 1 more tweak to <a> link colors with just a bump up in their brightness. Or, we set the config option to always underline links? What's your preference and thinking @wetneb ? I can do a PR to tweak and review tomorrow or next day.

[magdmartin]

looks good to me. I will merge the PR early next week if there are no other comments.

[magdmartin]

thanks for catching the typo @wetneb