build:contributor-docs now consults env vars for content repo & branch

[nbogie]

Changes:

1. build:contributor-docs (in contribute.ts) now allows overriding of the content repo and branch to clone through the env vars P5_REPO_URL and P5_BRANCH.

I matched this same pattern already in use by [build:reference]

2. Added a line of logging so it's clear which repo & branch are being considered / cloned

Example new usage:

P5_REPO_URL=../p5.js P5_BRANCH=my-branch-name npm run build:contributor-docs

Example new logging:

(omitted)

Building contributor docs...
Considering cloning repo: ../p5.js branch: fix-contrib-to-ref-guide-for-v2 into path: /omitted/p5.js-website/in/p5.js/
Recent version of library repo already exists, skipping clone...

Addresses #1114 (for main branch)