site: fix whimsy issues & remove defunct integrations

[NiklasMerz]

Motivation and Context

I came across the Apache Whimsy site check for projects and the site has some issues: https://whimsy.apache.org/site/project/cordova

Description

Fixed some links to pass Whimsy checks.

Some features do not work anymore as far as I can tell so I removed them:

1. Twitter feed on blog site
2. Doc search using algolia
3. Heroku slack invite

Do we want to keep and fix something?

Testing

I ran the site locally.

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary