Add copyright and authors list to reference manual

[sbrannen]

The copyright and authors list are both missing from the current versions of the Spring Framework reference manual.

https://docs.spring.io/spring/docs/5.2.0.M2/spring-framework-reference/

https://docs.spring.io/spring/docs/5.2.0.BUILD-SNAPSHOT/spring-framework-reference/

[sbrannen]

@Buzzardo, it appears that I cannot assign this issue to you.

[Buzzardo]

I don't know you wouldn't be able to assign it to me. Sorry about that.

I made this PR to fix it: #23049

[sbrannen]

Thanks for the quick turnaround!

I'll get that merged in.

Superseded by gh-23049