feat: add remark-math and rehype-katex to support latex math expression

[gyunseo]

This Pull Request resolves #150

• added remark-math@5.1.1 and rehype-katex@6.0.3 packages (the latest packages has some issues. please check out this issue)
• update astro.config.mjs to add remark-math in remark plugins
• update astro.config.mjs to add remark-katex in rehype plugins
• update Layout.astro to get katex stylesheet in head tag

the following image is what you get after changing above things.

[satnaing]

I mentioned about this feature in #150. So would you like to create a new PR for how-to blog post? I might co-author if required.

[satnaing]

I'll close this PR for now.
Thanks for your interest in contributing to AstroPaper.
If you wish, you can submit a PR for a 'how-to' post.

[gyunseo]

@satnaing I see! I'm going to make a new PR for how-to blog post.