Improve API Reference pages (#1415)

**Pull Request Checklist**
- [x] Fixes #1388. Closes #965 (as it's too nebulous to keep open, we
should make new specific issues for anything remaining)
- [x] Documentation/examples added
- [x] [Good commit messages](https://cbea.ms/git-commit/) and/or PR
title

**Description of PR**
Overhaul API reference docs:

* Remove the `hera.md` file expanding the `hera.workflows` module (this
was the page that loaded everything)
* Add new sections for the API docs
* Remove unused `admonition` extension
* Tinker with other mkdocstrings settings for best docs experience
* Update docstrings and add some object links and examples links

---------

Signed-off-by: Elliot Gunton <elliotgunton@gmail.com>

Author: elliotgunton

docs/api/events/models.md

@@ -3,3 +3,6 @@
 ## `hera.events.models`
 
 ::: hera.events.models.io.argoproj.events.v1alpha1
+    options:
+        inherited_members: false
+        show_source: false

docs/api/workflows/hera.md

@@ -1,5 +1,8 @@
-# Hera Workflows Auto-generated Models
+# Hera Workflows Auto-Generated Models
 
 ## `hera.workflows.models`
 
 ::: hera.workflows.models.io.argoproj.workflow.v1alpha1
+    options:
+        inherited_members: false
+        show_source: false

docs/api/workflows/models.md

@@ -0,0 +1,5 @@
+::: hera.workflows.archive
+    options:
+        inherited_members: false
+        show_source: false
+        show_if_no_docstring: false

docs/api/workflows/supporting_classes/archive.md

@@ -0,0 +1,15 @@
+::: hera.workflows.artifact
+    options:
+        show_if_no_docstring: false
+        members:
+        - Artifact
+        - ArtifactLoader
+        - ArtifactoryArtifact
+        - AzureArtifact
+        - GCSArtifact
+        - GitArtifact
+        - HDFSArtifact
+        - HTTPArtifact
+        - OSSArtifact
+        - RawArtifact
+        - S3Artifact

docs/api/workflows/supporting_classes/artifact.md

@@ -0,0 +1 @@
+::: hera.workflows.env

docs/api/workflows/supporting_classes/env.md

@@ -0,0 +1 @@
+::: hera.workflows.env_from

docs/api/workflows/supporting_classes/env_from.md

@@ -0,0 +1,17 @@
+# First-Party Volumes
+
+::: hera.workflows.volume
+    options:
+        head_level: 3
+        show_root_heading: False
+        members:
+        - EmptyDirVolume
+        - ExistingVolume
+        - ConfigMapVolume
+        - DownwardAPIVolume
+        - EphemeralVolume
+        - FlexVolume
+        - SecretVolume
+        - HostPathVolume
+        - NFSVolume
+        - ProjectedVolume

docs/api/workflows/supporting_classes/first_party_volumes.md

@@ -0,0 +1 @@
+::: hera.workflows.metrics

docs/api/workflows/supporting_classes/metrics.md

@@ -0,0 +1 @@
+::: hera.workflows.parameter