Add release notes for 2.6.0

[rupal-bq]

Description

Add release notes for 2.6.0

Issues Resolved

#1357

Check List

• New functionality includes testing.
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented.
  • New functionality has javadoc added
  • New functionality has user manual doc added
• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[codecov-commenter]

Codecov Report

Merging #1367 (db8c405) into main (b826835) will not change coverage.
The diff coverage is n/a.

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@@            Coverage Diff            @@
##               main    #1367   +/-   ##
=========================================
  Coverage     99.96%   99.96%           
  Complexity     2464     2464           
=========================================
  Files           220      220           
  Lines          5833     5833           
  Branches        359      359           
=========================================
  Hits           5831     5831           
  Misses            2        2           

Flag	Coverage Δ
sql-engine	99.96% <ø> (ø)

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[dai-chen]

023-02-22T17:29:00.7731753Z /home/runner/work/sql/sql/opensearch/src/main/java/org/opensearch/sql/opensearch/planner/physical/MLCommonsOperatorActions.java:107: error: cannot access ToXContentObject
2023-02-22T17:29:00.7732627Z       ColumnValue columnValue = row.getValue(i);
2023-02-22T17:29:00.7733116Z                                    ^
2023-02-22T17:29:00.7733620Z   class file for org.opensearch.common.xcontent.ToXContentObject not found

Shall we go ahead and merge the release notes first?

[dai-chen]

The PR is failing on the ML code above. Since this is only doc changes and 2.6 branch is not impacted, I just merged it.