Write flashing guide for the T440p

- Build on the testing instructions in https://github.com/osresearch/heads/pull/1282.
- Distinguish between compiling Heads yourself, compiling Heads yourself with your own blobs, and using the prebuilt CircleCI ROM.
- Make note of trouble with the speaker cable.
- Make note of delicate nature of the casing and show how I opened it "pulling outwards." The first pass, I broke a few tabs, and had to order a replacement casing.

Start by reviewing the testing instructions in pull request 1282 and determine where the T440p flashing guide belongs. Document the three build options, speaker-cable warning, and casing-opening technique, including the risk of broken tabs; the guide is done when all requested scenarios and cautions are covered.

Written by the indexing model from the issue text.