Contributor guide

[sharifsalah]

Adds an initial contributor guide for OpenCue to the site.

Here's a summary of the changes in this PR:

• Configures a new contributor top-level menu, alongside the docs and blog
• Adds layout files for the new section
• Adds an index file for the new section
• Adds a version of the CONTRIBUTOR guide from the OpenCue repo as it appeared in July **

** This is the default fall back if the steps in the script below don't complete. I didn't go for the very latest version as that would make it hard to test if the script was working.

Also updates the build script ++ to attempt to do the following:

• Clone the OpenCue repo
• Fetch the date for the latest revision of the CONTRIBUTOR guide
• Update the contributor guide for the site as well as the date

++ Some of the steps in the build script won't work in macOS due to differences between the versions of sed required by Netlify.

Outstanding issues:

• Some of the style of the Contributor guide doesn't quite match the rest of the site. For example, the site uses sentence case for headings and titles. I can follow up with a PR to update the source file in the OpenCue repo.
• Some of the links in the CONTRIBUTOR guide are relative (./CODEOWNERS), which won't work on the site. Not sure what the best way to address this, perhaps a FQDN link to the main OpenCue repo?

Direct link to the staged contributor guide:

https://deploy-preview-92--elated-haibt-1b47ff.netlify.com/contributing/opencue/contributing/

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Sharif Salah (35d445e, ea77679, 0830310)

[bcipriano]

Sorry for the delay here -- this looks great!

Make sure you've got the latest CONTRIBUTING version as we have made some changes to that file recently.

[sharifsalah]

Thanks! I checked and verified the staged version matches the latest commits in the OpenCue repo.