Add documentation for bun upgrade

[jsandford-mastery]

What is the type of issue?

Documentation is missing

What is the issue?

bun upgrade is not mentioned in the docs. There are no current mentions in gh issues about this adding this, but there are mentions there of using it. Good news is that it is in bun --help.

Where did you find it?

Found in https://bun.sh/docs/ and not found at https://bun.sh/docs/cli/upgrade or https://bun.sh/docs/cli/update.