Updates remaining links to account sections on this page instead (_for `accounts`, `aliases`, `quotas`_).

This page will cover the features and defer to separate pages for more specific content where relevant.

- Manual method not necessary to document.
- Condense `setup` example guidance.
- Quotas / Aliases content migrated to Overview when not specific about file provisioner.

Some of the content is this commit is not a complete revision.

Much better docs on the sub-addressing feature supported by Postfix and Dovecot, along with the guidance with usage in Sieve.

Add some context regarding DMS accounts and their distinction/overlap from the email address functionality, and it's relevant context for receiving/sending.

File provisioner, minor revisions to referencing associated config files and account management.

Better documented with links and coverage over the workaround details we've implemented.

Minor revisions with phrasing, admonitions for structure and better explanation of the feature functionality/purpose.

Extensively covers known issues and technical details that have been discussed often enough.

The improvements should benefit both users and maintainers.

This rewrite should more clearly document the feature, along with a better example and additional links for reference.

Minor update to this page:
- Links extracted to bottom of page as per convention.
- ENV file example converted to preferred `compose.yaml` ENV settings.

- Correct link to subaddressing section
- Make the config file example snippets intended filename less ambiguous.
- Minor rephrasng.

Revised the account section and added additional clarity for common confusion with relation to sender address and multi-domain support.

Top of the page now clarifies it's a technical reference and directs users to the related pages for configuration / caveats.

Technical Overview links to Dovecot docs were missing.

File based provisioner docs:
- Sections indent with info admonitions.
- Accounts section expanded with config format and example.
- Quotas section expanded and shifted to bottom (alphabetical sort).
- Split into `setup` CLI and config reference groups.

Overview page:
- Sections indent with info admonitions.
- Revised content.

This is related to accounts and aliases, but not provisioners, thus extract out of the accounts parent section.

This lacked documentation but was community contributed feature to allow further customization of a Dovecot Account.

It has caveats as DMS does not take these into consideration anywhere in scripts. Documenting officially for better awareness.

Better outline the OAuth2 login process, the two supported login mechanisms and their docs/rfcs, along with documenting caveat with mail client compatibility.

Add a verification tip for the OAuth2 support, showing how `curl` can be used, along with caveat presently affecting the `curl` in DMS v14.

Additionally note the feature still isn't documented fully, providing the user with additional references for more information.

`ACCOUNT_PROVISIONER` ENV docs minimized. No `OIDC` provisioner plans, the OAuth2 docs page now mentions SCIM 2.0 API as the next step towards resolving that concern. The tip admonition was removed as it no longer provides value, instead we link to the Account Management overview page.

Dovecot Master Accounts docs page now lightly document the `setup` CLI and config format for the feature.