I want to build documentation site using Jekyll and GitHub Pages. The problem is Jekyll only accept a filename under _posts
with exact pattern like YYYY-MM-DD
Don't use posts; posts are things with dates. Sounds like you probably want to use collections instead; you get all the power of Posts; but without the pesky date / naming requirements.
https://jekyllrb.com/docs/collections/
I use collections for almost everything that isn't a post. This is how my own site is configured to use collections for 'pages' as well as more specific sections of my site: