Restore additional "Browser Support" info (es5-shim etc.)

[seven-phases-max]

Per less/less.js#3211.
Just:

If you need to run less in an older browser, please use an es-5 shim which will add the javascript features that less requires.

would be enough.

Additional selfnote: also restore 2.x branch of the documentation to be able to reference any lost information (even if not important).

[matthew-dean]

Additional selfnote: also restore 2.x branch of the documentation to be able to reference any lost information (even if not important).

Not a bad idea. I didn't think about supporting past doc versions.