Incorporate accessibility recommendations

[zkat]

See https://dl.acm.org/doi/fullHtml/10.1145/3411764.3445544 (h/t @wraithgar).

A nice way to gate this is on the NO_GRAPHICS environment variable that miette uses to render narrated error reports. This'll need to be mapped to a global CLI flag that can also be configured through the CLI or config file.

Some thoughts on what this should mean:

1. Use the Narrated miette reporter
2. Remove loglevel prefixes from log lines
3. Disable colors
4. Disable emoji
5. Any time a progress bar would be rendered, replace this with a "starting X"..."finished X" pair, to notify that something is happening
6. Write a dedicated Accessibility page in the mdbook about what screenreader-mode is, what it does, and how to enable it.

This may also mean that we need to set up the miette handler hook ourselves, so we have more control over which handler gets used and how they're configured, rather than using miette's default.