Fleet-managed EA - Installation packages and Fleet-managed upgradability

[lucabelluccini]

At the moment, we document limitations about the fact a specific installation method can/cannot be upgraded in textual form:

• Containers (cannot be upgraded via Fleet) https://www.elastic.co/guide/en/fleet/current/elastic-agent-container.html#:~:text=When%20Elastic%20Agent%20runs%20inside%20a%20container%2C%20it%20cannot%20be%20upgraded%20through%20Fleet%20as%20it%20expects%20that%20the%20container%20itself%20is%20upgraded.
• DEB, RPM (here). There is a warning where we state the upgrade is not supported for DEB & RPM
• MSI (here). There is a confusing statement which it can trick users into thinking that you can upgrade an EA installed via MSI, while it's not possible (we have users reporting it's not possible)

Would be good to have a table with the installation package method and:

• Can be upgraded via Fleet: yes / no
• How to upgrade detailing the upgrade method for each package method (link to the page)

[pierrehilbert]

@ycombinator it would be great to have someone from your team working with @kilfoyle to update our doc with the last pieces of work we did (example: Deb/RPM).