Ignore md pages when indexing if noIndex keyword set true

[dmitrymatio]

Description

Added support for flagging a page as noIndex.
Search index operation will ignore pages with noIndex: true in frontmatter.
This is a simple feature but important for repos with transcluded pages under src/pages/ where we index all md pages by default. Currently such repos show duplicate results for the transcluded content in local search. The noIndex flag will help with removing these duplicates.

Related Issue

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.