[Site]: support structured schema (JSON-LD)

### Current State

**Description of the current state/issue**: For each page under the Apache APISIX Website, we don't provide a structured schema for search engines, so the search engine has to try to understand each page's content meaning. Structured Schema is an Advanced SEO solution, please refer to the following links for more information.


1. https://moz.com/blog/json-ld-for-beginners
2. https://developers.google.com/search/docs/advanced/structured-data/intro-structured-data
3. https://stackoverflow.com/questions/38670851/whats-a-script-type-application-ldjsonjsonobj-script-in-a-head-sec/38745339#38745339
4. https://support.google.com/knowledgepanel/answer/9163198?hl=zh-Hans&ref_topic=9164489
5. https://developers.google.com/search/docs/advanced/guidelines/get-started

### Desired State

Validator Tools: https://validator.schema.org/