[Doc] add doc for ngram index

[before-Sunrise]

Why I'm doing:

What I'm doing:

Fixes #issue

What type of PR is this:

• [ ] BugFix
• [ ] Feature
• [ ] Enhancement
• [ ] Refactor
• [ ] UT
• [x] Doc
• [ ] Tool

Does this PR entail a change in behavior?

• [ ] Yes, this PR will result in a change in behavior.
• [x] No, this PR will not result in a change in behavior.

If yes, please specify the type of change:

• [ ] Interface/UI changes: syntax, type conversion, expression evaluation, display information
• [ ] Parameter changes: default values, similar parameters but with different default values
• [ ] Policy changes: use new policy to replace old one, functionality automatically enabled
• [ ] Feature removed
• [ ] Miscellaneous: upgrade & downgrade compatibility, etc.

Checklist:

• [ ] I have added test cases for my bug fix or my new feature
• [ ] This pr needs user documentation (for new or modified features or behaviors)
  • [ ] I have added documentation for my new feature or new function
• [ ] This is a backport pr

Bugfix cherry-pick branch check:

• [x] I have checked the version labels which the pr will be auto-backported to the target branch
  • [ ] 3.2
  • [ ] 3.1
  • [ ] 3.0
  • [ ] 2.5

[DanRoscigno]

@mergify backport branch-3.3

[mergify[bot]]

backport branch-3.3

✅ Backports have been created

• #45124 [Doc] add doc for ngram index (backport #40841) has been created for branch branch-3.3 but encountered conflicts

[github-actions[bot]]

[FE Incremental Coverage Report]

:white_check_mark: pass : 0 / 0 (0%)

[github-actions[bot]]

[BE Incremental Coverage Report]

:white_check_mark: pass : 0 / 0 (0%)