Add email form to landing page and sub pages

Summary:
As requested from Christabel, adding an email collecting form to the site using Mailerlite.

Inserted their script in the head and added the form to the landing page, releases page and docs page

Test Plan: Submit some test emails and confirm the form is working as intended with Christabel

Reviewers: #bitcoin_abc, johnkuney, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: jasonbcox, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8420