Custom Upstream Composer.lock Exclude Instructions

**Issue background:** For composer custom upstreams, it is crucial that composer.lock not be included in the repository, but it also cannot be gitignored, confusing developers and leading to inadvertent git conflicts. Explicit instructions for how to approach this as a developer are included in the [Integrated Composer section of the docs](https://docs.pantheon.io/guides/integrated-composer/ic-upstreams#custom-upstream-and-site-structure) but not in the [primary Custom Upstream documentation](https://docs.pantheon.io/guides/custom-upstream/create-custom-upstream). **Proposed change:** This specific instruction would be helpful in the primary [Custom Upstream guides](https://docs.pantheon.io/guides/custom-upstream/create-custom-upstream): - and warrants a stronger callout. <img width="898" height="334" alt="Image" src="https://github.com/user-attachments/assets/f8582fa6-a2a7-4546-8a73-0326ac130733" />