Improve WFDB documentation

[shah-in-boots]

Would like to have the documentation for the wfdb functions to be atomic and shared/reusable.

Should split reading WFDB files into their components

• [x] Reading in signal data
• [x] Reading in header data
• [x] Reading in annotation data

Will also need to reference how to install and utilize WFDB until the entire code-base can be shifted over to R directly.

• [ ] Vignette on installing WFDB
• [ ] Vignette on guide to using WFDB functions

[shah-in-boots]

Still need to improve documentation of annotations.