Document Alpha features

Today, we have alpha features that are enabled by setting
`enable-api-fields` to `"alpha"`. This is informed in each
of the feature's documentation, but there's no central place
where all these feeatures are listed so users have to dig through
the docs to find out all the alpha features they'd be enabling.

In this change, we add a cental documentation for all the Alpha features
with references to their TEPs and Releases. If applicable, we add the
individual feature flag as well.

docs/alpha-features.md

@@ -0,0 +1,29 @@
+<!--
+---
+linkTitle: "Alpha Features"
+weight: 5000
+---
+-->
+
+# Alpha Features
+
+- [Introduction](#introduction)
+- [Alpha Table](#alpha-features-table)
+
+## Introduction
+
+This doc provides a list of features in Tekton Pipelines that are considered [Alpha](https://github.com/tektoncd/pipeline/blob/main/api_compatibility_policy.md#alpha-beta-and-ga).
+
+Set `enable-api-fields` to `"alpha"` to allow alpha features to be used.
+
+## Alpha Features Table
+
+| Feature                                                                                                                         | TEP                                                                                         | Release                                                              | Individual Flag             |
+|:------------------------------------------------------------------------------------------------------------------------------- |:------------------------------------------------------------------------------------------- |:-------------------------------------------------------------------- |:--------------------------- |
+| [Bundles ](https://github.com/tektoncd/pipeline/blob/main/docs/pipelineruns.md#tekton-bundles)                                  | [TEP-0005](https://github.com/tektoncd/community/blob/main/teps/0005-tekton-oci-bundles.md) | [v0.18.0](https://github.com/tektoncd/pipeline/releases/tag/v0.18.0) | `enable-tekton-oci-bundles` |
+| [`Runs` and `Custom Tasks`](https://github.com/tektoncd/pipeline/blob/main/docs/runs.md)                                        | [TEP-0002](https://github.com/tektoncd/community/blob/main/teps/0002-custom-tasks.md)       | [v0.19.0](https://github.com/tektoncd/pipeline/releases/tag/v0.19.0) | `enable-custom-tasks`       |
+| [Isolated `Workspaces`](https://github.com/tektoncd/pipeline/blob/main/docs/workspaces.md#isolated-workspaces)                  | [TEP-0029](https://github.com/tektoncd/community/blob/main/teps/0029-step-workspaces.md)    | [v0.24.0](https://github.com/tektoncd/pipeline/releases/tag/v0.24.0) |                             |
+| [Hermekton](https://github.com/tektoncd/pipeline/blob/main/docs/hermetic.md)                                                    | [TEP-0025](https://github.com/tektoncd/community/blob/main/teps/0025-hermekton.md)          | [v0.25.0](https://github.com/tektoncd/pipeline/releases/tag/v0.25.0) |                             |
+| [Specifying `onError` for a `Step`](https://github.com/tektoncd/pipeline/blob/main/docs/tasks.md#specifying-onerror-for-a-step) | [TEP-0040](https://github.com/tektoncd/community/blob/main/teps/0040-ignore-step-errors.md) | [v0.27.0](https://github.com/tektoncd/pipeline/releases/tag/v0.27.0) |                             |
+| [Implicit `Parameters`](https://github.com/tektoncd/pipeline/blob/main/docs/taskruns.md#implicit-parameters)                    | [TEP-0023](https://github.com/tektoncd/community/blob/main/teps/0023-implicit-mapping.md)   | [v0.28.0](https://github.com/tektoncd/pipeline/releases/tag/v0.28.0) |                             |
+

docs/install.md

@@ -363,8 +363,8 @@ The default is `false`. For more information, see the [associated issue](https:/
 use of custom tasks in pipelines.
 
 - `enable-api-fields`: set this flag to "stable" to allow only the
-most stable features to be used. Set it to "alpha" to allow alpha
-features to be used.
+most stable features to be used. Set it to "alpha" to allow [alpha
+features](./alpha-features.md) to be used.
 
 - `scope-when-expressions-to-task`: set this flag to "true" to scope `when` expressions to guard a `Task` only. Set it
   to "false" to guard a `Task` and its dependent `Tasks`. It defaults to "false". For more information, see [guarding