feat(ux): add CMD+K support

[itsdevbear]

Problem

One of the biggest missing features of mdbook, imo.

Proposed Solution

No response

Notes

No response

[ehuss]

Can you clarify what this issue means? What is "CMD+K support"?

[itsdevbear]

search / spotlight

[chriskrycho]

For a good summary of the pattern, see Maggie Appleton’s Command K Bars write-up. The full pattern often has features beyond just search, and is more of a “command” palette. (“K” for “command but with a K” because Command C on Mac and Control C on Windows and Linux are taken for the “Copy” command.)