Migrate the BTextMailComponent docs to the Haiku Book.

Migrate the BMailComponent docs to the Haiku Book.

I've updated the docs to match the current BMailComponent, documented
new functions, and cleaned up the MailComponent.h file to at least
somewhat m

Migrate the BMailComponent docs to the Haiku Book.

I've updated the docs to match the current BMailComponent, documented
new functions, and cleaned up the MailComponent.h file to at least
somewhat match our coding style.

First in a series (there are 3 more old API docs on the Mail Kit in that
"Public API" folder.)

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docs/apps/mail: Extract the "Writing Add-ons" guide and convert to markdown.

I guess it was at one point in Be Styled Text format, but it didn't
extract that way for me, so I had to manually recreat

docs/apps/mail: Extract the "Writing Add-ons" guide and convert to markdown.

I guess it was at one point in Be Styled Text format, but it didn't
extract that way for me, so I had to manually recreate the styling...

show more ...

docs: Remove old "Mail Preferences" documentation.

Actually based on old BeOS documentation and long surpassed by
the Haiku User Guide.

DiskUsage docs: update screenshots.

- Why were we using old BeOS screenshots here?
- Why is this not part of the user guide anyways?