Add example to import variables into Less file

[zkwsk]

Checklist

• Includes tests
• Update documentation

Changes proposed in this pull request:

I was a little puzzled that I could not import colors into my less file, until I realized that the example given in the documentation was only for Sass, so I added an example for Less.

Reviewers should focus on:

Whether this adheres to your documentation style.

[palantirtech]

Thanks for your interest in palantir/blueprint, @zkwsk! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[adidahiya]

simple and straightforward, thanks!