Add documentation link

Could you please add the documentation for version 2.1.1 to this link?
https://choreonoid.org/en/documents/index.html

We have not yet been able to produce an English version of the manual for recent versions. We are very sorry about this.

Thank you for letting me know. I understand that translating the manual can take some time. I was able to read through the Japanese documents using google translate. Thanks again