docs: add note about adding new models in alphabetical order

[fyliu]

Overview

We previously discussed how two PRs with new models at the end of the file can cause a merge conflict. It's mostly avoidable by having a system to add the code in different places in the file, such as alphabetical ordering.

Action Items

• add a note about using alphabetical ordering in docs/contributing/howto/add-model-and-api-endpoints.md

Resources/Instructions

• Add the note to docs/contributing/howto/add-model-and-api-endpoints.md