Documentation changes

[lgrn]

• markdownlint, for more information see: https://github.com/DavidAnson/markdownlint/tree/main/doc

• add step asking users to verify key fingerprint rather than including the entire key.

• automatically fill out DISTRO and ARCH and write output to repo file instead of copy-pasting

• move key section up as it's a prerequisite for later steps

[lgrn]

@stgraber with 225f796dacd1dba08d5d5d39f375e60e70ff0ab9 I think/hope I've incorporated all the feedback so far, have a look.

[stgraber]

Just a few more minor fixes to do.

Also, can squash your commits together and have the resulting commit signed-off (Signed-off-by: Firstname Lastname <email> in the commit message).

[lgrn]

@stgraber all right, have a look.

[stgraber]

Thanks!