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 <[email protected]>

usage/clone-filter.en-us.md

@@ -0,0 +1,39 @@
+---
+date: "2021-02-02"
+title: "Clone filters (partial clone)"
+slug: "clone-filters"
+sidebar_position: 25
+draft: false
+toc: false
+aliases:
+  - /en-us/clone-filters
+menu:
+  sidebar:
+    parent: "usage"
+    name: "Clone filters"
+    sidebar_position: 25
+    identifier: "clone-filters"
+---
+
+# Clone filters (partial clone)
+
+Git introduces `--filter` option to `git clone` command, which filters out
+large files and objects (such as blobs) to create partial clone of a repo.
+Clone filters are especially useful for large repo and/or metered connection,
+where full clone (without `--filter`) can be expensive (as all history data
+must be downloaded).
+
+This requires Git version 2.22 or later, both on the Gitea server and on the
+client. For clone filters to work properly, make sure that Git version
+on the client is at least the same as on the server (or later). Login to
+Gitea server as admin and head to Site Administration -> Configuration to
+see Git version of the server.
+
+By default, clone filters are enabled, unless `DISABLE_PARTIAL_CLONE` under
+`[git]` is set to `true`.
+
+See [GitHub blog post: Get up to speed with partial clone](https://github.blog/2020-12-21-get-up-to-speed-with-partial-clone-and-shallow-clone/)
+for common use cases of clone filters (blobless and treeless clones), and
+[GitLab docs for partial clone](https://docs.gitlab.com/ee/topics/git/partial_clone.html)
+for more advanced use cases (such as filter by file size and remove
+filters to turn partial clone into full clone).