Add stub of identity page

Create an initial page for the identity / authorization page to
live in.

doc/development/agent/identity.md

+---
+stage: Configure
+group: Configure
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers
+---
+
+# Kubernetes Agent identity and authentication **(PREMIUM ONLY)**
+
+This page uses the word `agent` to describe the concept of the
+GitLab Kubernetes Agent. The program that implements the concept is called `agentk`.
+Read the
+[architecture page](https://gitlab.com/gitlab-org/cluster-integration/gitlab-agent/-/blob/master/doc/architecture.md)
+for more information.
+
+## Agent identity and name
+
+In a GitLab installation, each agent must have a unique, immutable name. This
+name must be unique in the project the agent is attached to, and this name must
+follow the [DNS label standard from RFC 1123](https://tools.ietf.org/html/rfc1123).
+The name must:
+
+- Contain at most 63 characters.
+- Contain only lowercase alphanumeric characters or `-`.
+- Start with an alphanumeric character.
+- End with an alphanumeric character.
+
+Kubernetes uses the
+[same naming restriction](https://kubernetes.io/docs/concepts/overview/working-with-objects/names/#dns-label-names)
+for some names.
+
+The regex for names is: `/\A[a-z0-9]([-a-z0-9]*[a-z0-9])?\z/`.
+
+## Multiple agents in a cluster
+
+A Kubernetes cluster may have 0 or more agents running in it. Each agent likely
+has a different configuration. Some may enable features A and B, and some may
+enable features B and C. This flexibility enables different groups of people to
+use different features of the agent in the same cluster.
+
+For example, [Priyanka (Platform Engineer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#priyanka-platform-engineer)
+may want to use cluster-wide features of the agent, while
+[Sasha (Software Developer)](https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/#sasha-software-developer)
+uses the agent that only has access to a particular namespace.
+
+Each agent is likely running using a
+[`ServiceAccount`](https://kubernetes.io/docs/tasks/configure-pod-container/configure-service-account/),
+a distinct Kubernetes identity, with a distinct set of permissions attached to it.
+These permissions enable the agent administrator to follow the
+[principle of least privilege](https://en.wikipedia.org/wiki/Principle_of_least_privilege)
+and minimize the permissions each particular agent needs.
+
+## Kubernetes Agent authentication
+
+When adding a new agent, GitLab provides the user with a bearer access token. The
+agent uses this token to authenticate with GitLab. This token is a random string
+and does not encode any information in it, but it is secret and must
+be treated with care. Store it as a `Secret` in Kubernetes.
+
+Each agent can have 0 or more tokens in GitLab's database. Having several valid
+tokens helps you rotate tokens without needing to re-register an agent. Each token
+record in the database has the following fields:
+
+- Agent identity it belongs to.
+- Token value. Encrypted at rest.
+- Creation time.
+- Who created it.
+- Revocation flag to mark token as revoked.
+- Revocation time.
+- Who revoked it.
+- A text field to store any comments the administrator may want to make about the token for future self.
+
+Tokens can be managed by users with `maintainer` and higher level of
+[permissions](../../user/permissions.md).
+
+Tokens are immutable, and only the following fields can be updated:
+
+- Revocation flag. Can only be updated to `true` once, but immutable after that.
+- Revocation time. Set to the current time when revocation flag is set, but immutable after that.
+- Comments field. Can be updated any number of times, including after the token has been revoked.
+
+The agent sends its token, along with each request, to GitLab to authenticate itself.
+For each request, GitLab checks the token's validity:
+
+- Does the token exist in the database?
+- Has the token been revoked?
+
+This information may be cached for some time to reduce load on the database.
+
+## Kubernetes Agent authorization
+
+GitLab provides the following information in its response for a given Agent access token:
+
+- Agent configuration Git repository. (The agent doesn't support per-folder authorization.)
+- Agent name.