improve the Plugins page

[liuxiran]

Also, @foolwc nice of you to make this page mobile responsive, wasn't earlier to this.

One thing that I think needs improvement is that long scroll to reach the later section on mobile. We will need to add a system like that in the docs (check this page through mobile-device view, devtools - docs) for jumping across sections or something similar that does this.

But that can be done in some later PR...

Originally posted by @1502shivam-singh in https://github.com/apache/apisix-website/issues/501#issuecomment-904649338