Documentation: process: Update email client instructions for Thunderbird

Documentation: process: Update email client instructions for Thunderbird

The instructions don't match with the current Thunderbird interface.
Clarification on using external extensions.
New information on how to avoid writing HTML emails.
Tell user to restart Thunderbird after modifications.

Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Sotir Danailov <sndanailov@gmail.com>
Link: https://lore.kernel.org/r/20220715211307.9358-1-sndanailov@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>