[DOCS]: Create new docs repository instructions

[narcher7]

Description

In order to add a new section to elastic.co, users must either add their book to a different elastic repository or create a new docs repository.

Either option requires a similar but slightly divergent set of steps. Some of the repo addition processes are documented in the "Creating a new book" page in the docs section of the wiki, but there's no information about configuring a new repository.

Verify the steps required to add a book to an existing Elastic repository and add the additional steps needed to create your own docs repository.

Acceptance Test Criteria

1. Add a docs directory with the required files to an existing elastic repo.

OR

1. Create a new repository with a docs directory and the required files.

2. Modify the conf.yaml and shared attributes files in the elastic/docs repo to call your existing or new repo.

3. (For new repositories), add your new repository to elastic/infra repo, in order to get the benefits of Elastic's CI/CD system Jenkins.

Notes

• Add the "Team:Docs" label to new issues.
• Be sure to add any necessary screenshots for clarity.
• Include any conditions or caveats that may affect customers.