Docs: Add guide on converting polyrepo to monorepo (#18903)

[Ashish-coder-gif]

Hi there! This pull request adds a new guide on how to convert a polyrepo to a monorepo using Pants, as requested in issue #18903.
The new document covers the benefits of migrating, explains how Pants' features make it possible, and provides a conceptual walkthrough of the process.

Closes #18903

[sureshjoshi]

Hi, thanks for the PR. I'm not sure how relevant that Github ticket is anymore, but in any case - what was your experience with taking one of your polyrepo projects and moving it into a monorepo and using Pants?

Any Github repos I can look at?

What hurdles did you run into?

[Ashish-coder-gif]

Hi @sureshjoshi! Thanks a lot for taking the time to review my PR 😊

I put this guide together after going through the discussion in issue #18903 and learning about the general process of migrating from a polyrepo to a monorepo using Pants.

I haven’t personally done a large-scale migration yet, but my goal was to make the guide easy to follow for newcomers—highlighting the main benefits, challenges, and steps based on what I found in community discussions and the Pants documentation.

I’d be happy to add an example repo or include more practical details if you think that would make it more useful!