Restructure and move Git content (#18336)

* Rename map topic

* Delete map topic

* Rename map topic

* Add redirect

* Remove last two map topics

* Move article on adding a remote into a long-form guide

* Move article on changing a remote's URL

* Move articles on renaming and removing remotes

* Move remote URL article

* Fix typo

* Remove category index file

* Move a few articles to the new category

* Move a few more files

* Move managing remote repos article

* Move the rest of the getting started with git map topic

* Move the first half of the using git map topic

* Move the rest of the articles and 🔥 the directory

* Fix failing test

* Remove Using Git from product index

* Apply suggestions from code review

Co-authored-by: Laura Coursen <lecoursen@github.com>

* Apply suggestions from code review

Co-authored-by: Laura Coursen <lecoursen@github.com>

content/github/getting-started-with-github/about-git-subtree-merges.md

@@ -0,0 +1,110 @@
+---
+title: About Git subtree merges
+redirect_from:
+  - /articles/working-with-subtree-merge/
+  - /subtree-merge/
+  - /articles/about-git-subtree-merges
+  - /github/using-git/about-git-subtree-merges
+intro: 'If you need to manage multiple projects within a single repository, you can use a *subtree merge* to handle all the references.'
+versions:
+  free-pro-team: '*'
+  enterprise-server: '*'
+  github-ae: '*'
+---
+
+Typically, a subtree merge is used to contain a repository within a repository. The "subrepository" is stored in a folder of the main repository.
+
+The best way to explain subtree merges is to show by example. We will:
+
+- Make an empty repository called `test` that represents our project
+- Merge another repository into it as a subtree called `Spoon-Knife`.
+- The `test` project will use that subproject as if it were part of the same repository.
+- Fetch updates from `Spoon-Knife` into our `test` project.
+
+### Setting up the empty repository for a subtree merge
+
+{% data reusables.command_line.open_the_multi_os_terminal %}
+2. Create a new directory and navigate to it.
+  ```shell
+  $ mkdir test
+  $ cd test
+  ```
+3. Initialize a new Git repository.
+  ```shell
+  $ git init
+  > Initialized empty Git repository in /Users/octocat/tmp/test/.git/
+  ```
+4. Create and commit a new file.
+  ```shell
+  $ touch .gitignore
+  $ git add .gitignore
+  $ git commit -m "initial commit"
+  > [main (root-commit) 3146c2a] initial commit
+  >  0 files changed, 0 insertions(+), 0 deletions(-)
+  >  create mode 100644 .gitignore
+  ```
+
+### Adding a new repository as a subtree
+
+1. Add a new remote URL pointing to the separate project that we're interested in.
+  ```shell
+  $ git remote add -f spoon-knife git@github.com:octocat/Spoon-Knife.git
+  > Updating spoon-knife
+  > warning: no common commits
+  > remote: Counting objects: 1732, done.
+  > remote: Compressing objects: 100% (750/750), done.
+  > remote: Total 1732 (delta 1086), reused 1558 (delta 967)
+  > Receiving objects: 100% (1732/1732), 528.19 KiB | 621 KiB/s, done.
+  > Resolving deltas: 100% (1086/1086), done.
+  > From git://github.com/octocat/Spoon-Knife
+  >  * [new branch]      main     -> Spoon-Knife/main
+  ```
+2. Merge the `Spoon-Knife` project into the local Git project. This doesn't change any of your files locally, but it does prepare Git for the next step.
+
+  If you're using Git 2.9 or above:
+  ```shell
+  $ git merge -s ours --no-commit --allow-unrelated-histories spoon-knife/main
+  > Automatic merge went well; stopped before committing as requested
+  ```
+
+  If you're using Git 2.8 or below:
+  ```shell
+  $ git merge -s ours --no-commit spoon-knife/main
+  > Automatic merge went well; stopped before committing as requested
+  ```
+3. Create a new directory called **spoon-knife**, and copy the Git history of the `Spoon-Knife` project into it.
+  ```shell
+  $ git read-tree --prefix=spoon-knife/ -u spoon-knife/main
+  ```
+4. Commit the changes to keep them safe.
+  ```shell
+  $ git commit -m "Subtree merged in spoon-knife"
+  > [main fe0ca25] Subtree merged in spoon-knife
+  ```
+
+Although we've only added one subproject, any number of subprojects can be incorporated into a Git repository.
+
+{% tip %}
+
+**Tip**: If you create a fresh clone of the repository in the future,  the remotes you've added will not be created for you. You will have to add them again using [the `git remote add` command](/github/getting-started-with-github/managing-remote-repositories).
+
+{% endtip %}
+
+### Synchronizing with updates and changes
+
+When a subproject is added, it is not automatically kept in sync with the upstream changes. You will need to update the subproject with the following command:
+
+```shell
+$ git pull -s subtree <em>remotename</em> <em>branchname</em>
+```
+
+For the example above, this would be:
+
+```shell
+$ git pull -s subtree spoon-knife main
+```
+
+### Further reading
+
+- [The "Advanced Merging" chapter from the _Pro Git_ book](https://git-scm.com/book/en/v2/Git-Tools-Advanced-Merging)
+- "[How to use the subtree merge strategy](https://www.kernel.org/pub/software/scm/git/docs/howto/using-merge-subtree.html)"