Support helm migration from current grafana/loki chart (#6958)

<!--  Thanks for sending a pull request!  Before submitting:

1. Read our CONTRIBUTING.md guide
2. Name your PR as `<Feature Area>: Describe your change`.
  a. Do not end the title with punctuation. It will be added in the changelog.
  b. Start with an imperative verb. Example: Fix the latency between System A and System B.
  c. Use sentence case, not title case.
  d. Use a complete phrase or sentence. The PR title will appear in a changelog, so help other people understand what your change will be.
3. Rebase your PR if it gets out of sync with main
-->

**What this PR does / why we need it**:

These are changes to the helm chart and documentation need to migration from the current version of the `grafana/loki` helm chart to the new one in this repo.

The current plan (as outlined in the README) is to support 3 upgrade paths:

1. From the existing `grafana/loki` chart using filesystem storage. This path will result in a single binary deployment using local filesystem storage with the new chart. This PRs includes changes to make sure this path works.
2. ~From the existing `grafana/loki` chart using object storage. This path will result in an SSD deployment with 1 read and 1 write), and has yet to be tested/documented.~ (Edit: I no longer think we need to support this use case)
3. From the existing `grafana/loki-simple-scalable` chart using object storage. This path will result in an SSD reployment with 3 read and 3 write replicas, and has yet to be tested/documented.

By clarifying the three modes, and that single binary will only support a single, filesystem backed instance, it made possible some refactors. For example, the user no longer needs to specify a `deploymentMode`, and we were able to reorganize the templates a bit into a flatter folder structure.

**Checklist**
- [X] Documentation added
- [ ] Tests updated (will do in subsequent PR)
- [ ] Is this an important fix or new feature? Add an entry in the `CHANGELOG.md`.
- [ ] Changes that require user attention or interaction to upgrade are documented in `docs/sources/upgrading/_index.md`

Co-authored-by: Karsten Jeschkies <k@jeschkies.xyz>