Convert HTML tables to Markdown tables

[StephenBrown2]

Using python-markdown syntax via sphinx-markdown-tables extension.

I noticed that some of the formatting wasn't being applied in the tables, and figured it was probably because of the inline html, markdown processing was being suspended.

This converts all the <table>s to markdown tables, so that the formatting will work, and also improves the readability and fixes the markup (The bluetooth tethering table was particularly poor markup).

HTML tags can still be used in the table cells, as with <br>s in the OPENAPS_ lines in Nightscout setup.

While testing this locally, I used the make serve command, which errored as there is no build command, and I also added the build/ folder to .gitignore.