Add Mintlify automations

[hahnbeelee]

Description

This PR adds the automations necessary to create the statsforecast Mintlify docs. When a push to main is made, the build-docs workflow will run which will upload the needed Mintlify docs files at the docs branch.

This PR also fixes minor formatting errors

Steps needed before merging

Reference steps: https://github.com/Nixtla/statsforecast/pull/671

Maintenance instructions

When a new page is added it needs to be added to mint.json.

Test plan

• Install the Mintlify CLI (Prerequisite: node.js, if you have a mac I like to use nvm
• Run bash ./preview-mintlify-docs.bash to get a preview of the docs locally. Note - the preview will only show mlforecast but the Nixtla deployment will be a frankenstein merged version of all 4 docs instances. I created this command so that developers can preview & sanity check their changes before pushing.

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