Set :idprefix: and :idseparator: in adoc files

Currently, the "getting started" links in the project README successfully link
to the `#getting-started` section on GitHub, but fail to link on
developers.yubico.com because the heading ID there is `#_getting_started`
instead. These settings should resolve this.

README

@@ -3,6 +3,8 @@ java-webauthn-server
 :toc:
 :toc-placement: macro
 :toc-title:
+:idprefix:
+:idseparator: -
 
 image:https://github.com/Yubico/java-webauthn-server/workflows/build/badge.svg["Build Status", link="https://github.com/Yubico/java-webauthn-server/actions"]
 image:https://img.shields.io/endpoint?url=https%3A%2F%2FYubico.github.io%2Fjava-webauthn-server%2Fcoverage-badge.json["Mutation test coverage", link="https://Yubico.github.io/java-webauthn-server/"]

doc/Migrating_from_v1.adoc

@@ -1,4 +1,6 @@
 = v1.x to v2.0 migration guide
+:idprefix:
+:idseparator: -
 
 The `2.0` release of the `webauthn-server-core` module
 removes some deprecated features

webauthn-server-attestation/README.adoc

@@ -2,6 +2,8 @@
 :toc:
 :toc-placement: macro
 :toc-title:
+:idprefix:
+:idseparator: -
 
 An optional module which extends link:../[`webauthn-server-core`]
 with a trust root source for verifying

webauthn-server-attestation/doc/Migrating_from_v1.adoc

@@ -1,4 +1,6 @@
 = v1.x to v2.1 migration guide
+:idprefix:
+:idseparator: -
 
 The `2.0` release of the `webauthn-server-attestation` module
 makes lots of breaking changes compared to the `1.x` versions.

webauthn-server-core/README

@@ -1,4 +1,6 @@
 = Web Authentication server library
+:idprefix:
+:idseparator: -
 
 Implementation of a Web Authentication Relying Party (RP).
 

webauthn-server-demo/README

@@ -1,4 +1,6 @@
 = webauthn-server-demo
+:idprefix:
+:idseparator: -
 
 A simple self-contained demo server supporting multiple authenticators per user.
 It illustrates how to use the required integration points, the most important of