docs: fix broken links and clarify myAPI variable in README

[mixelburg]

Fixes two README issues reported separately:

Fixes #281 — The API documentation links were pointing to apidevtools.com/swagger-parser/docs/ (returns 404). This PR updates all the broken /docs/ paths to the correct working URLs:

• Feature links: docs/swagger-parser.html#... → swagger-parser.html#...
• Circular refs anchor: docs/#circular-refs → #circular-refs
• Main docs page: docs/ → /

Fixes #280 — The example code snippets used a bare myAPI variable with no context on what it represents (a file path, a URL, an object?). Added a comment and variable declaration const myAPI = "./path/to/my-api.yaml"; above each example to make it immediately clear.