Add a soft SLA for reviewers and maintainers

What does this MR do?

Set expectations for both reviewers and contributors on when changes are reviewed.

The wording is rather soft I think, by design. From personal experience I know I can review anything assigned to me within a working day. But its hard to judge what the load is on others and I think this is a valuable iteration as is too.

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• [-] Tests added for this feature/bug
• [-] Tested in all supported browsers
• [-] Conforms to the code review guidelines
• [-] Conforms to the merge request performance guidelines
• [-] Conforms to the style guides
• [-] Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• [-] Security reports checked/validated by reviewer