[DOC] Add FILLNULL PPL documentation

[normanj-bitquill]

What do you want to do?

• 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#3075

Version:
2.19

What other resources are available?
opensearch-project/sql#3075
opensearch-project/opensearch-spark#723

[kkhatua]

@normanj-bitquill
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

[kkhatua]

Thank you, @anasalkouz . We'll mark it as 3.0.0 so that we dont miss it in the next cycle. @penghuo need not wait until the next release to update the documentation.

[opensearch-ci-bot]

Hi @normanj-bitquill
,

As part of the entrance criteria, all the documentation pull requests need to be drafted and in technical review.

Since there is no pull request linked to this issue, please take one of the following actions:

• Create the pull request and link it to this issue.
  
• If you already have a pull request created, please link it to this issue.
  
• If this feature is not targeted for the currently labeled release version, please update the issue with the correct release version.
  

Please note: Missing documentation can block the release and cause delays in the overall process.

Thank you!

[kkhatua]

@anasalkouz , @penghuo .... do we have the PR for this ready for review?