[DOC] Add trendline PPL documentation

[jduo]

What do you want to do?
Add documentation for the new trendline PPL command.

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request.
Add documentation for the new trendline PPL command implemented in PR opensearch-project/sql#3071 .

*Version:
2.19

What other resources are available?
opensearch-project/sql#3071
opensearch-project/sql#3013

[kkhatua]

@jduo
Can you link the doc PR to this issue? It is blocking the release for 2.19 launch.

[anasalkouz]

There is no hard requirement to update website documentation at the current moment. @penghuo will share an issue that we will do one time effort to update all PPL commands into website documentation

[acarbonetto]

Trendline was not backported to 2.x (2.19) release.
It's in the 3.x release only.
Unless that changes on the implementation, we can update this issue and mark it part of the 3.x release instead.