Update GraphQL reference to prepare for move to Hugo

What does this MR do and why?

Two changes to prepare the GraphQL page for the move to Hugo:

1. Renames the markdown file for the GraphQL reference page. For the Hugo site, index pages must be named _index.md rather than index.md.
2. Moves the page title from an h1 into a title front matter attribute, also required for Hugo.

The renamed index file also requires updating all links that pointed to the index.md file.

We are handling this page separately because it is auto-generated. Other pages will be updated as part of a larger scripted migration.

The current Nanoc website and /help are both already set up to work with both of these changes.

• Review app for Nanoc: https://main-ee-178234.docs.gitlab-review.app/ee/api/graphql/reference/
• Review app for Hugo: https://new.docs.gitlab.com/upstream-review-mr-ee-178234/api/graphql/reference/ (note that shortcodes, such as icons or alert boxes, will not render until we merge !178795 (merged) at launch time)

References

• https://gitlab.com/gitlab-org/technical-writing-group/gitlab-docs-hugo/-/issues/168
• https://gitlab.com/gitlab-org/technical-writing-group/gitlab-docs-hugo/-/issues/82

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