Add imap-send to the list of email helpers

[AdityaGarg8]

Changes

• This PR adds the link of git imap-send docs to the Email category on the main page.

Context

git imap-send is a git command that enables a person to send an email in mbox format generated by git format-patch to a folder using IMAP.

Its link was missing from https://git-scm.com/docs