Upgrade to latest mdbook 0.5.*

[bronius]

Currently in mdbook 0.4.*, and in #42 it was discovered that github action calling latest giving mdbook v0.5.0 broke builds.

Test building the site in mdbook 0.5.0 and update both relevant github workflow actions to use the latest once it's all found working. See the mdbook v0.5 migration guide for breaking changes. Add the options that retain the current look and feel (ie, disable the new sidebar headers nav). Confirm that the site builds without error in v0.5.0 before committing.

At least one difference is in 0.5.0, mdbook build will currently fail due to mdbook-admonish linking a non-existent file. In this case (and others like it), take the most logical, necessary action (like remove the unlinked file or link it in an obvious manner), and clearly list the changes needed to be made in order to satisfy this latest mdbook version. Anticipate and overcome other non-joyous fallout.

[bronius]

Attempted in #51 with the most basic copilot requesting .. . and it went downhill.

There's no urgency here, but if we want to pick it up, consider starting at HEAD^ in #51 copilot/upgrade-mdbook-to-0-5-0