Add support for generating other file extensions in MarkBind Sites

[gerteck]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• DevOps
• Improve developer experience
• Others, please explain:

Overview of changes:

Addresses #2761

Enhances by adding functionality for MarkBind to generate other file types while retaining nunjucks variable functionality, i.e. from .md files into .XXX files that might not be .html files.

Summary of Changes

• Added an optional fileExtension property that can be specified in site.json > pages.
• createPage method updated to use the fileExtension to determine output file path.
  • Additionally, it was refactored: previously, output file path created by directly reusing the page’s source path, assuming a clean, relative path. New change computes the path relative to the site root and normalize it before generating the output path, so that all output files are placed in the correct location under the output directory, even if the source path is absolute or contains .., and makes the behavior consistent across operating systems.
• Update passing along the fileExtension property through the build pipeline.
• Rename outputPageHtml to writeOutputFile to reflect the new process.

Anything you'd like to highlight/discuss:

TODO:

• Add documentation updates

Testing instructions:

Proposed commit message: (wrap lines at 72 characters)

Add generation of other file extension types

TODO

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Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No unrelated changes

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Reviewer checklist:

Indicate the SEMVER impact of the PR:

• Major (when you make incompatible API changes)
• Minor (when you add functionality in a backward compatible manner)
• Patch (when you make backward compatible bug fixes)

At the end of the review, please label the PR with the appropriate label: r.Major, r.Minor, r.Patch.

Breaking change release note preparation (if applicable):

• To be included in the release note for any feature that is made obsolete/breaking

Give a brief explanation note about:

• what was the old feature that was made obsolete
• any replacement feature (if any), and
• how the author should modify his website to migrate from the old feature to the replacement feature (if possible).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.65%. Comparing base (024b2ce) to head (32d6a32).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2762   +/-   ##
=======================================
  Coverage   61.65%   61.65%           
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  Files         130      130           
  Lines        7167     7167           
  Branches     1480     1480           
=======================================
  Hits         4419     4419           
  Misses       2685     2685           
  Partials       63       63           

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