Add entry in PR description asking Pull Requesters to document any new commands they add

[moskyb]

Description

The current (internal) guidance is that when a buildkite employee makes a new release of the agent, they open a PR against the docs that includes updates in CLI reference there, which is kinda-sorta autogenerated from the helptexts that we write for commands in this repo.

the issue with this process is that the process for adding a new page for a new agent cli command is a bit involved, and often it leaves the person who made the release in charge of adding docs for a command they didn't write, which is a minor pain.

this PR adds another couple of checkboxes to the PR description (the one i'm typing in right now!) asking that if the user has added a command, that they open a PR against the docs repo. i also added a checkbox for "updated the readme as necessary", because the other checkbox was looking lonely on its own

Context

buildkite/docs#2710

Testing

• Tests have run locally (with go test ./...). Buildkite employees may check this if the pipeline has run automatically.
• Code is formatted (with go fmt ./...)

[triarius]

The docs repo is public too yeah? I think this document belongs there as most of the commands assume the working directory is a docs repo.

1. Clone or change direcotry to the docs repo: https://github.com/buildkite/docs.git
2. Follow the instructions in https://github.com/buildkite/docs?tab=readme-ov-file#agent

I suggested putting it in an agent section in the README as there is no docs directory in the docs repo for documentation on the documentation! You could create one if you want. I think it's a good precedent to set.