Add configurable file suffix for generated markdown files

[jdillard]

Motivation

This PR adds support for customizing the file extension of generated markdown files to support conventions like https://llmstxt.org/, which suggests using the .html.md naming pattern (e.g., index.html.md).

Previously, the output file suffix was hardcoded to .md, which meant users couldn't generate files with alternative extensions

Summary of Changes

• Added markdown_file_suffix config value (default: .md)
• Created dedicated test_custom_file_suffix() to verify files are generated with correct extension
• Updated _chmod_output() helper to handle both .md and .html.md files

Relates to #36

[coveralls]

Pull Request Test Coverage Report for Build 18484897391

Details

• 2 of 2 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.003%) to 99.103%

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Totals
Change from base Build 13610142503:	0.003%
Covered Lines:	773
Relevant Lines:	780

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💛 - Coveralls

[jacobtomlinson]

This is really helpful thanks!

[liran-funaro]

LGTM