Quick fixes to guide users

[hemidactylus]

A set of quick improvements to better guide the user and help them not feeling lost in the current website experience:

• a prominent link, in the home page, to a representative colab-available langchain tutorial
• clearer text in the "look for colab button" phrasing
• clarify what users have to do to later run the colabs in the "api setup" page
• clarify "you should have cloned the repo" before the otherwise obscure reference to a dir in repo
• more context on the "create a virtual env" part

[hemidactylus]

done, merged to main