Split Security and Privacy considerations sections

[beaufortfrancois]

As advised by speced/bikeshed#2120 and https://w3ctag.github.io/security-questionnaire/#reviews, Web Bluetooth spec should split the current "Security and Privacy considerations" section in two separate Security and Privacy considerations sections.

  [INFO] Build bikeshed document "index.bs" (common)…
    $ bikeshed --die-on=warning spec "index.bs" "index.bs.built.html" 
      LINE 339: W3C policy requires Privacy Considerations and Security Considerations to be separate sections, but you appear to have them combined into one.
       ✘  Did not generate, due to fatal errors
    Failed