doc: update caprover documentations to address the following

ref: https://discord.com/channels/647412545203994635/704008423468957707/895926998755581983

> Caprover install on DigitalOcean is pretty straight forward. But from there on it's a nightmare to get the Chatwoot to behave in a proper way. I will list down the 2-3 days struggle here. Still no success.
> 1. After the one-click app install and all the DNS settings all done and verified, immediately go for an upgrade to latest docker versions.
> 2. Then access rails console to enable super_admin interface,
> 3. Then enable email_channel, inbound_emails etc. (lots of search in docs for self hosted chatwoot, and tried everything that makes sense )
> the environment variable settings are another saga. Somehow most of it is now working. except conversation continuity and email channel. Figured out Postmark config, and I can see inbound emails being processed. The webhook to chatwoot from there is also tested to be working. The "check" gives http 204 status.
> What now?
> Only Sendgrid is supported or any documentaton for postmark too?
> Will the active storage work without any .env setting explicitly?
> In the Cprover deployment log, I have a warning: [Warning] One or more build-args [SECRET_KEY_BASE POSTGRES_HOST POSTGRES_USERNAME MAILER_SENDER_EMAIL VAPID_PRIVATE_KEY RAILS_LOG_TO_STDOUT POSTGRES_PASSWORD REDIS_URL REDIS_PASSWORD FRONTEND_URL RAILS_INBOUND_EMAIL_PASSWORD VAPID_PUBLIC_KEY MAILER_INBOUND_EMAIL_DOMAIN RAILS_ENV ACTIVE_STORAGE_SERVICE POSTGRES_DATABASE SMTP_ADDRESS SMTP_PASSWORD RAILS_INBOUND_EMAIL_SERVICE POSTMARK_INGRESS_API_KEY SMTP_USERNAME] were not consumed
> Is that serous?

Improve the documentation for caprover