add design doc for Yarn 1.x support

[taylormadore]

You can see a barebones PoC with just prefetch (No SBOM generation) using the offline mirror here: https://github.com/taylormadore/cachi2/tree/yarn-v1-limited-poc

Maintainers will complete the following section

• [x] Commit messages are descriptive enough
• [-] Code coverage from testing does not decrease and new code is covered
• [-] Docs updated (if applicable)
• [-] Docs links in the code are still valid (if docs were updated)

Note: if the contribution is external (not from an organization member), the CI pipeline will not run automatically. After verifying that the CI is safe to run:

• approve GitHub Actions workflows by clicking a button
• approve the Red Hat Trusted App Pipeline container build by commenting /ok-to-test (as is the standard for Pipelines as Code)

[taylormadore]

Since V1:

• Non-substantive edits to wording
• Added a section describing how users will specify the Yarn (v1 and v2+) package manager: https://github.com/containerbuildsystem/cachi2/pull/585#discussion_r1700431511
• Added PURL examples
• Added a section for follow-up actions

[taylormadore]

Since V2:

• Added some links to the Yarn codebase for undocumented CLI options and environment variables
• Added a small section describing how undocumented Yarn environment variables work