consolidate usage of placeholder text

**Is your feature request related to a problem? Please describe.**

examples:

- [`backend/api/~apiname~/schema.graphql`](https://docs.amplify.aws/cli-legacy/graphql-transformer/overview/#update-schema)
- [*amplify/backend/api/YOUR-API-NAME/build*](https://docs.amplify.aws/cli-legacy/graphql-transformer/overview/#api-category-project-structure) (on the same page, italicized and not inline code block)
- [`AUTH__USERPOOLID`](https://docs.amplify.aws/cli-legacy/graphql-transformer/function/#usage)
- [`amplify:`](https://docs.amplify.aws/cli/function/build-options/)
- [`https://.execute-api.eu-west-2.amazonaws.com/dev/items`](https://docs.amplify.aws/cli/restapi/restapi/)
- [`https://{restapi-id}.execute-api.{region}.amazonaws.com/{environment}/{path}.`](https://docs.amplify.aws/cli/restapi/restapi/#create-a-rest-api) (same page)

[This](https://docs.amplify.aws/cli/restapi/restapi/#set-up-a-rest-api-with-amazon-dynamodb) is not an example of placeholder text that will need to be consolidated:

[Here](https://docs.amplify.aws/cli/auth/override/) is an example of the proposed, standardized placeholder text

```
amplify/backend/auth//
```

**Describe the solution you'd like**

using an inline code block, refer to placeholder text with angle brackets, lowercase, kebab-case. When referring to files in the `amplify` directory, _always_ preface with `amplify/`

```md
refer to your schema at `amplify/backend/api//schema.graphql`
```

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
Add any other context or screenshots about the feature request here.