Generate all doc pages as Markdown format

[flovntp]

Generate all of the pages in a markdown format (LLMS)
To get any page in Markdown format, just change .html in your browser by .md
example :
https://docs.upsun.com.llms-md-pages-xzf6x5y-ucq44jg6ofare.eu-5.platformsh.site/administration/web/configure-project.md

Please note:
for the homepage, that's different as index.html is not displayed, so add index.md at the end:
https://docs.upsun.com.llms-md-pages-xzf6x5y-ucq44jg6ofare.eu-5.platformsh.site/index.md

I also

• change the links in the llms.txt file to point on this new markdown version of doc pages.
• reformat replaceRE function in a single partial file and use it everywhere we need markdown output

Why

Closes #{ISSUE_NUMBER}

What's changed

Where are changes

Updates are for:

• platform (sites/platform templates)
• upsun (sites/upsun templates)