doc improvement for Winlogbeat upgrade

**Describe the enhancement:**
Today, all Beats share a common [upgrade guide](https://www.elastic.co/guide/en/beats/libbeat/current/upgrading.html).
While it's easy to maintain and avoids duplicates, it misses some nuances when it comes to Winlogbeat specifically.

It'll be better if we can have dedicated Winlogbeat upgrade doc or at least add Winlogbeat specific notes to the existing doc.

A few feedbacks from the users:

- It's not clear whether it's required to remove the older version before the upgrade (both major/minor upgrade).
- It's also not totally clear what to backup before the upgrade (not "data and config directories" as mentioned in current common upgrade guide).

Start with the linked shared Beats upgrade guide and review how Winlogbeat upgrades are currently described. Clarify whether older versions must be removed and exactly what users should back up; done means Winlogbeat-specific upgrade guidance answers both questions, either in the shared guide or a dedicated document.

Written by the indexing model from the issue text.