add a doc on the logit bias trick

[lostmygithubaccount]

First check

• I added a descriptive title to this issue.
• I used the GitHub search to look for a similar issue and didn't find it.
• I searched the Marvin documentation for this feature.

Describe the current behavior

I want to reference the logit bias trick sometimes; in particular, in an upcoming Ibis blog

currently, the best reference is the Twitter thread. it's also mentioned on the classification docs page with a link to that Twitter thread, but I think it'd be better to have a conceptual document or something on the Marvin website explaining the trick

Describe the proposed behavior

add a doc for the logit bias trick (or add a dedicated section explaining it in the classification doc)

Example Use

linking to it from Slack, blogs, etc.

Additional context

it's an awesome trick :)

[Dev-Khant]

@zzstoatzz @jlowin Can pick this up? Do I need to create a separate section for it or just elaborate it more in the current section itself?