Docusaurus-ify (#26051)

This PR cleans up the docs in a way to make them simpler to ingest by
our [docs repo](https://gitea.com/gitea/gitea-docusaurus).

1. It includes all of the sed invocations our ingestion did, removing
the need to do it at build time.
2. It replaces the shortcode variable replacement method with
`@variable@` style, simply for easier sed invocations when required.
3. It removes unused files and moves the docs up a level as cleanup.

---------

Signed-off-by: jolheiser <john.olheiser@gmail.com>

1 files changed, 46 insertions, 0 deletions

diff --git a/docs/content/usage/incoming-email.en-us.md b/docs/content/usage/incoming-email.en-us.md
new file mode 100644
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+---
+date: "2022-12-01T00:00:00+00:00"
+title: "Incoming Email"
+slug: "incoming-email"
+sidebar_position: 13
+draft: false
+toc: false
+aliases:
+  - /en-us/incoming-email
+menu:
+  sidebar:
+    parent: "usage"
+    name: "Incoming Email"
+    sidebar_position: 13
+    identifier: "incoming-email"
+---
+
+# Incoming Email
+
+Gitea supports the execution of several actions through incoming mails. This page describes how to set this up.
+
+## Requirements
+
+Handling incoming email messages requires an IMAP-enabled email account.
+The recommended strategy is to use [email sub-addressing](https://en.wikipedia.org/wiki/Email_address#Sub-addressing) but a catch-all mailbox does work too.
+The receiving email address contains a user/action specific token which tells Gitea which action should be performed.
+This token is expected in the `To` and `Delivered-To` header fields.
+
+Gitea tries to detect automatic responses to skip and the email server should be configured to reduce the incoming noise too (spam, newsletter).
+
+## Configuration
+
+To activate the handling of incoming email messages you have to configure the `email.incoming` section in the configuration file.
+
+The `REPLY_TO_ADDRESS` contains the address an email client will respond to.
+This address needs to contain the `%{token}` placeholder which will be replaced with a token describing the user/action.
+This placeholder must only appear once in the address and must be in the user part of the address (before the `@`).
+
+An example using email sub-addressing may look like this: `incoming+%{token}@example.com`
+
+If a catch-all mailbox is used, the placeholder may be used anywhere in the user part of the address: `incoming+%{token}@example.com`, `incoming_%{token}@example.com`, `%{token}@example.com`
+
+## Security
+
+Be careful when choosing the domain used for receiving incoming email.
+It's recommended receiving incoming email on a subdomain, such as `incoming.example.com` to prevent potential security problems with other services running on `example.com`.