[RFC] Gitbook and document enhancement

[TianlongLiang]

Most of the current GitBook documents were created about three years ago. WAMR has evolved a lot in the past three years; it's a good idea to review the current content and enhance it to reflect the up-to-date WAMR features and usage.

The enhancement tasks I can think of now:

• Add gitbook document for new samples
• Add gitbook document for new features
• Update description of
• Add CI action to validate links, maybe use remark-validate-links or lychee-action. Depends on whether we want to check http link in addition to the markdown link

[TianlongLiang]

GitBook-style relative links are references within the repo(files/anchors), remark-validate-links is good enough. Both internal + external link checking should use lychee