Fix PulseSensorPlayground documentation links

[yury-g]

What changed

• identifies this repository as the current official Arduino library and links the two recommended starter examples
• corrects the Arduino Library Manager search term to PulseSensor Playground
• rebuilds the example index from the 19 example directories that actually ship in the library
• replaces seven references to the removed PulseSensor_BPM_Alternative.ino with a working support destination and a request for board/core details

Why

The README and example index sent new users to a wrong Library Manager query and ten nonexistent example paths. Seven sketches also recommended a removed fallback example when timer initialization failed. These dead ends interrupt the first-project and troubleshooting journey.

User impact

New users get the exact install term, a current first-project path, and an example index whose links all resolve. Unsupported-board failures now point to the repository's Issues page with the details maintainers need.

Validation

• git diff --check
• verified library.properties declares name=PulseSensor Playground
• confirmed all 19 example directories are represented exactly once in the index
• resolved every new relative Markdown link against the checkout
• confirmed the GitHub Issues destination returns HTTP 200
• confirmed no PulseSensor_BPM_Alternative or old "PulseSensor.com search references remain

No repositories were archived and no runtime behavior changed.