[FW][IMP] email marketing: update essentials doc

[fw-bot]

PROJECT TASK: https://www.odoo.com/web#id=3699244&cids=3&menu_id=4720&action=333&active_id=3835&model=project.task&view_type=form

Complete update of the Email Marketing doc.
Updated toctree slightly, so the Email Marketing Essentials (formerly Email Marketing) appears beneath the category selection.
Added a link to tutorial on the 'Email Marketing' category page

Forward-Port-Of: #7521

[robodoo]

Pull request status dashboard.

[fw-bot]

@ksc-odoo @samueljlieber cherrypicking of pull request #7521 failed.

stdout:

Auto-merging content/applications/marketing/email_marketing.rst
CONFLICT (content): Merge conflict in content/applications/marketing/email_marketing.rst

stderr:

22:21:52.485669 git.c:463               trace: built-in: git cherry-pick 474a8bd9ab2905933cec5b1dc34e2205c478cfa5
error: could not apply 474a8bd9a... [IMP] email marketing: update essentials doc
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
----------
status:

Either perform the forward-port manually (and push to this branch, proceeding as usual) or close this PR (maybe?).

In the former case, you may want to edit this PR message as well.

⚠️ after resolving this conflict, you will need to merge it via @robodoo.

More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port

[samueljlieber]

Resolving this conflict will need to wait until #8463 (Lost Leads) finishes merging into master

[samueljlieber]

@robodoo r+