Improve about page

[math-GH]

After:

Changes proposed in this pull request:

• added: link to the documentation
• added: external links will open in new tab
• added: information about environment in data/config.php (shown for admins,Shown only, when it is not production. I thank that it is important additionally to the version number. )

How to test the feature manually:

1. go to config -> about page
2. see the page

check as guest user (not logged in) too:

1. click on about FreshRSS on the login screen
2. see the page

Pull request checklist:

• clear commit messages
• code manually tested
• unit tests written (optional if too hard)
• documentation updated

Additional information can be found in the documentation.