Add Lerp as an imputation method and update the docs config

[WenjieDu]

What does this PR do?

1. fixing Request to add linear interpolation imputation method as a new naive model #457;
2. fixing Docs configs should be updated by requested #460;

Before submitting

• This PR is made to fix a typo or improve the docs (you can dismiss the other checks if this is the case).
• Was this discussed/approved via a GitHub issue? Please add a link to it if that's the case.
• I have commented my code, particularly in hard-to-understand areas.
• I have written necessary tests and already run them locally.