Multi-line editing and other updates

[NotTheDr01ds]

The primary purpose of this PR is to add information on multi-line editing as mentioned in #1671.

Of course, as often happens, I rabbit-trailed into other fixes and updates along the way:

• Reedline: Added table-of-contents
• Reedline: Added multi-line editing section
• Reedline: Added a number of missing keybindings to Emacs and Vi modes
• Reedline: Formatting keybindings using <kbd> tags
• Reedline: Miscellaneous wordsmithing
• Reedline: Removed section on configuring the Prompt, as this is covered in the Configuration chapter. Added link.
• Quick Tour: Added link to Multi-line Editing
• Thinking in Nu: Added link to Multi-line Editing

[sholderbach]

Thanks for updating that!