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>

docs/content/usage/merge-message-templates.en-us.md

@@ -0,0 +1,53 @@
+---
+date: "2022-08-31T17:35:40+08:00"
+title: "Merge Message templates"
+slug: "merge-message-templates"
+sidebar_position: 15
+toc: false
+draft: false
+aliases:
+  - /en-us/merge-message-templates
+menu:
+  sidebar:
+    parent: "usage"
+    name: "Merge Message templates"
+    sidebar_position: 15
+    identifier: "merge-message-templates"
+---
+
+# Merge Message templates
+
+## File names
+
+Possible file names for PR default merge message templates:
+
+- `.gitea/default_merge_message/MERGE_TEMPLATE.md`
+- `.gitea/default_merge_message/REBASE_TEMPLATE.md`
+- `.gitea/default_merge_message/REBASE-MERGE_TEMPLATE.md`
+- `.gitea/default_merge_message/SQUASH_TEMPLATE.md`
+- `.gitea/default_merge_message/MANUALLY-MERGED_TEMPLATE.md`
+- `.gitea/default_merge_message/REBASE-UPDATE-ONLY_TEMPLATE.md`
+
+## Variables
+
+You can use the following variables enclosed in `${}` inside these templates which follow [os.Expand](https://pkg.go.dev/os#Expand) syntax:
+
+- BaseRepoOwnerName: Base repository owner name of this pull request
+- BaseRepoName: Base repository name of this pull request
+- BaseBranch: Base repository target branch name of this pull request
+- HeadRepoOwnerName: Head repository owner name of this pull request
+- HeadRepoName: Head repository name of this pull request
+- HeadBranch: Head repository branch name of this pull request
+- PullRequestTitle: Pull request's title
+- PullRequestDescription: Pull request's description
+- PullRequestPosterName: Pull request's poster name
+- PullRequestIndex: Pull request's index number
+- PullRequestReference: Pull request's reference char with index number. i.e. #1, !2
+- ClosingIssues: return a string contains all issues which will be closed by this pull request i.e. `close #1, close #2`
+
+## Rebase
+
+When rebasing without a merge commit, `REBASE_TEMPLATE.md` modifies the message of the last commit. The following additional variables are available in this template:
+
+- CommitTitle: Commit's title
+- CommitBody: Commits's body text