RFC: Document how to add (pre/post) buildpacks to build order

[jromero]

Summary

Based on this RFC, we now have two ways to prepend or append buildpacks to the builder order.

What questions should this documentation answer?

• How do I add buildpacks to the builder order using project.tom?
• How do I add buildpacks to the builder order via the CLI arguments?

Who is the target audience for this documentation?

• App Developer
• Buildpack Author
• Operator
• Other:

Additional Information

• Blocked by RFC: Support build.pre.buildpacks build.post.buildpacks in project.toml pack#1100
• Blocked by RFC: Support for --pre-buildpack and --post-buildpack pack#1099
• We may want to remove any references to from=builder if we have any.

[edmorley]

@AidanDelaney This one also isn't fixed and needs reopening:
https://buildpacks.io/docs/reference/config/project-descriptor/

[Ayush9026]

/assign i will solve this.