HTML fixes (#188)

Fix  #187 - a big bunch of HTML fixes in order to get valid HTML / clean results from https://validator.w3.org/nu/?doc=https%3A%2F%2Fartemis-nerds.github.io%2Fprotocol-docs%2F 
Specifically:

- `<content>` is apparently no longer a thing? Really? Converted to `<div>`
- The above required a corresponding `script.js` fix too
- `<a target="_new">` should apparently now be `<a target="_blank">`
- Some `<ul>` moved outside `<p>`
- Some `<dl> / <dt> / <dd>` structure fixed
- Unfortunately **A LOT** of indentation fixed because of the above - see diffs without whitespace
- This may cause merge conflicts with some of my other pending changes - will fix case-by-case
- Apparently bad practice to change number of cols in a table? Added some empty `<td>`
- Fixed the `toggleredalert` duplicate to the correct `toggleshields` - obsoletes #182 / fixes #181 
- `alt` added to all `<img>`
- rebased / resolved merge conflicts

Author: NoseyNick