Do not index HTML comments in Markdown documents

[solimant]

This PR omits HTML comments in Markdown documents from being indexed for search.

Description

In certain circumstances, Markdown documents may contain HTML comments, such as:

<!-- Comment 1 -->
Some text between comments
<!-- Comment 2 -->
Some more text

Indexing those comments is undesirable since those comments don't end up in the final rendered document.

This PR omits such comments so that only the target content is included in the search index.

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.