[DOC] Adding guides to explain UDF serialization and Broadcast variable usage

[Niharikadutta]

We are excited to review your PR.

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• There's a descriptive title that will make sense to other developers some time from now.
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• Your change description explains what the change does, why you chose your approach, and anything else that reviewers should know.
• You have included any necessary tests in the same PR.

[Niharikadutta]

@bamurtaugh Could you please take a look at the following guides I wrote to explain the UDF serialization behavior and Broadcast variable usage? Thank you for your time!

[bamurtaugh]

These are great, thanks so much for working on them @Niharikadutta 😄

[Niharikadutta]

@imback82 , @bamurtaugh Have added a section for UDF serialization and addressed all remaining comments. Please feel free to take a look and let me know your thoughts. Thanks!

[bamurtaugh]

LGTM, thanks @Niharikadutta!

[Niharikadutta]

@imback82 Please let me know if this looks ok or if there are any changes/suggestions you have. Thanks!

[imback82]

few nit comments, but LGTM in general.

[Niharikadutta]

@imback82 addressed remaining comments, please let me know if it looks ok. Thanks!