[Docs] Restructure of Git Best Practices doc (#7231)

* Add "Resynchronizing an Out-of-Sync Git Branch" doc

* Update (Git) Best Practices doc title and filename

* Rename Branch Resync doc

* fork Best Practices doc into multiple files

* Add the doc list to Git Best Practices doc

* Update sidebar

* Update internal references

* Update sidebar - add subsection

* Update Your Fork's Master page title

* title case on Git Best Practices main doc

* ... and in the Resynchronizing a Branch doc

* Please read Part 1

I worked really hard on this, okay?

* Please use branches, too.

* suggestions by mtei

* change note about adding multiple files

* note that the name given the remote repo is arbitrary

* suggestions by fauxpark

* Git Best Practices -> Best Git Practices

Reads more naturally.

* rephrase hint block regarding remote name

* rework the resynchronization instructions per mtei

* use hint boxes for reference to Part 1 doc

I may be addicted to hint boxes. I'm sorry fauxpark. :cry:

* add some clarity about the upstream repo

* wordsmithing per mtei

* restyle the shell code blocks

Makes them more consistent to the other docs in this section.

1 files changed, 71 insertions, 0 deletions

diff --git a/docs/newbs_git_resynchronize_a_branch.md b/docs/newbs_git_resynchronize_a_branch.md
new file mode 100644
index 000000000..2e6b26e09
--- /dev/null
+++ b/docs/newbs_git_resynchronize_a_branch.md
@@ -0,0 +1,71 @@
+# Resynchronizing an Out-of-Sync Git Branch
+
+Suppose you have committed to your `master` branch, and now need to update your QMK repository. You could `git pull` QMK's `master` branch into your own, but GitHub will tell you that your commit is a number of commits ahead of `qmk:master`, which can create issues if you want to make a pull request to QMK.
+
+?> This document builds upon the concepts detailed in [Your Fork's Master: Update Often, Commit Never](newbs_git_using_your_master_branch.md). If you are not familiar with that document, please read it first, then return here.
+
+## Backing Up the Changes on Your Own Master Branch (Optional)
+
+No one wants to lose work if it can be helped. If you want to save the changes you've already made to your `master` branch, the simplest way to do so is to simply create a duplicate of your "dirty" `master` branch:
+
+```sh
+git branch old_master master
+```
+
+Now you have a branch named `old_master` that is a duplicate of your `master` branch.
+
+## Resynchronizing Your Branch
+
+Now it's time to resynchronize your `master` branch. For this step, you'll want to have QMK's repository configured as a remote in Git. To check your configured remotes, run `git remote -v`, which should return something similar to:
+
+```sh
+QMKuser ~/qmk_firmware (master)
+$ git remote -v
+origin  https://github.com/<your_username>/qmk_firmware.git (fetch)
+origin  https://github.com/<your_username>/qmk_firmware.git (push)
+upstream        https://github.com/qmk/qmk_firmware.git (fetch)
+upstream        https://github.com/qmk/qmk_firmware.git (push)
+```
+
+If you only see one fork referenced:
+
+```sh
+QMKuser ~/qmk_firmware (master)
+$ git remote -v
+origin  https://github.com/qmk/qmk_firmware.git (fetch)
+origin  https://github.com/qmk/qmk_firmware.git (push)
+```
+
+add a new remote with:
+
+```sh
+git remote add upstream https://github.com/qmk/qmk_firmware.git
+```
+
+Then, redirect the `origin` remote to your own fork with:
+
+```sh
+git remote set-url origin https://github.com/<your_username>/qmk_firmware.git
+```
+
+Now that you have both remotes configured, you need to update the references for the upstream repository, which is QMK's, by running:
+
+```sh
+git fetch upstream
+```
+
+At this point, resynchronize your branch to QMK's by running:
+
+```sh
+git reset --hard upstream/master
+```
+
+These steps will update the repository on your computer, but your GitHub fork will still be out of sync. To resynchronize your fork on GitHub, you need to push to your fork, instructing Git to override any remote changes that are not reflected in your local repository. To do this, run:
+
+```sh
+git push --force-with-lease
+```
+
+!> **DO NOT** run `git push --force-with-lease` on a fork to which other users post commits. This will erase their commits.
+
+Now your GitHub fork, your local files, and QMK's repository are all the same. From here you can make further needed changes ([use a branch!](newbs_git_using_your_master_branch.md#making-changes)) and post them as normal.