Update links to Anaconda documentation

[charlottejmc]

The anaconda documentation has undergone a slight change, so a few of the links used in our lessons flag as broken.

• https://docs.anaconda.com/free/anaconda/getting-started/ -> https://docs.anaconda.com/getting-started/
• https://docs.anaconda.com/free/anaconda/ -> https://docs.anaconda.com/anaconda/
• https://docs.anaconda.com/free/anaconda/install/ -> https://docs.anaconda.com/anaconda/install/