Draft: gitaly: Stop manually templating Gitaly configuration

What does this MR do?

We'd like to stop manually templating each key/value pair destined to be rendered into the Gitaly config.toml file. This MR does this in a few steps:

1. Introduces a new config key to the Gitaly values.yaml that will be directly translated into the content of Gitaly's config.toml file. Users will specify Gitaly config directly as YAML, which will get translated into TOML during rendering.
2. Migrates subsets of configuration keys into the new config key, while preserving manual templating, placeholder values, and default values. Tests are updated to work with the new locations of the keys. camelCased keys are replaced with train_cased equivalents. Configuration semantics are updated to match Omnibus and direct config.toml configuration.
3. Replaces manual templating with a YAML to TOML translation.
4. Removes redundant documentation, pointing users to https://docs.gitlab.com/ee/administration/gitaly/configure_gitaly.html as the source of truth.

Related issues

https://gitlab.com/gitlab-org/gitaly/-/issues/6384

Author checklist

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Required

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• Merge Request Title and Description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline.
• Documentation created/updated.
• Tests added/updated, and test plan for scenarios not covered by automated tests.
• Equivalent MR/issue for omnibus-gitlab opened.

Reviewers checklist

• MR has a green pipeline on https://gitlab.com/gitlab-org/charts/gitlab.
• Consider downstream impact to the Operator, as per evaluating impact from changes to GitLab Chart.