[Meta] Create API reference templates

[KOTungseth]

In the future, all APIs deserve reference material. For customers to effectively use the APIs, they need reference material.

• Choose a place for a API reference template to exist, ideally a public location (Docs repo, shared folder)

• Choose best practices from existing or other content

Good API reference page has:
* Title
* Short description/Intro
* Endpoint/Request
* Description (more detailed)
* Endpoint/Request
* Path parameters
* Query parameters
* Request body
* Response body

Some APIs also require:
* Authorization/security details
* Definition lists
* Examples
* Default values

• Draft template for API page
  * Something that is useful, helpful to customers
  * Can be generated using tools that we have
  * Not too much glitter

• Draft template for definition page

Tests:

• [DOCS] Tests API template against rollup APIs elasticsearch#43402
• [DOCS] Rewrite create index API to test new API reference format elasticsearch#43427
• [DOCS] Rewrite delete index API to use new API reference format elasticsearch#43459
• [DOCS] Rewrite get index API to use new API reference format elasticsearch#43503
• [DOCS] Rewrite index exists API to use new API reference format elasticsearch#43546
• [DOCS] Updates data frame APIs to use API template elasticsearch#43610
• [DOCS] Updates ML APIs to use new API template elasticsearch#43711
• [DOCS] Reformat freeze unfreeze APis to use new API format elasticsearch#43948
• [DOCS] Reformat CCR APIs to use new API format elasticsearch#43952