doc: Improve wal_level and effective_wal_level GUC around logical replication

Hi,

In evaluating the proposed change to wal_level docs in [1]/messages/by-id/CA+v5N42vKtY17653eBHFx+Cw06E2fSmPrXmX2qKYG23=4Ycd7A@mail.gmail.com I found it to be
undesirable, but the discussion and my own review discovered some other
changes that should be considered.

Since effective_wal_level now exists, Logical Replication Getting Started
needs to be updated.
Add links to the main topics that the different levels apply to.
Be explicit about the ordering of minimal, replica, and logical.
Move the behavior of effective_wal_level to the GUC for it, leaving behind
just the pointer that wal_level is now just a "minimum".

David J.

[1]: /messages/by-id/CA+v5N42vKtY17653eBHFx+Cw06E2fSmPrXmX2qKYG23=4Ycd7A@mail.gmail.com
/messages/by-id/CA+v5N42vKtY17653eBHFx+Cw06E2fSmPrXmX2qKYG23=4Ycd7A@mail.gmail.com