Improve previews for links to documentation

[wilkinsona]

I recently linked someone to https://docs.spring.io/spring-security/reference/migration/servlet/session-management.html#_require_explicit_saving_of_securitycontextrepository on Gitter and the preview was this:

Session Management Migrations :: Spring Security

6.0.2 6.1.0-SNAPSHOT 6.1.0-M1 6.0.3-SNAPSHOT 6.0.2 6.0.1 6.0.0 6.0.0-RC2 6.0.0-RC1 6.0.0-M7 6.0.0-M6 6.0.0-M5 6.0.0-M4 6.0.0-M3 6.0.0-M2 6.0.0-M1 5.8.3-SNAPSHOT 5.8.2 5.8.1 5.8.0 5.8.0-RC1

The title's fine but the content isn't very helpful. AIUI, an improvement could be made with some open graph meta tags in the of each page.