Newsletter Subscribe

[nikhilchauhangithub]

Description of your changes

Add a feature of newsletter subscribe via MailChimp "Fixes #801":

I have:

• [ x] Read and followed KubeVela's contribution process.
• [x ] Update sidebar.js if adding a new page.
• [ x] Run yarn start to ensure the changes has taken effect.

Special notes for your reviewer

The official account and official mail should be used to deliver mail and for the API key,

[chivalryq]

Who is hosting the Mailchimp's service? Does Mailchimp have free plan for open source project? @nikhilchauhangithub

[chivalryq]

Sorry but I have to revert the change first and we can keep working on this until everything is clear.