Document extension extraction process (#19085)

Release Notes:

- N/A

extensions/EXTRACTION.md

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+# Extracting an extension to dedicated repo
+
+These are some notes of how to extract an extension from the main zed repository and generate a new repository which preserves the history as best as possible.  In the this example we will be extracting the `ruby` extension, substitute as appropriate.
+
+## Pre-requisites
+
+Install [git-filter-repo](https://github.com/newren/git-filter-repo/blob/main/INSTALL.md):
+
+```
+brew install git-filter-repo
+```
+
+## Process
+
+1. Create an expressions.txt file somewhere (e.g. `~/projects/expressions.txt`)
+
+```
+ruby: ==>
+extension: ==>
+chore: ==>
+zed_extension_api: ==>
+regex:(?<![\[a-zA-Z0-9])(#[0-9]{3,5})==>zed-industries/zed\1
+```
+
+This file takes the form of `patern==>replacement`, where the replacement is optional.
+Note whitespace matters so `ruby: ==>` is removing the `ruby:` prefix from a commit messages and adding a space after `==> ` means the replacement begins with a space.  Regex capture groups are numbered `\1`, `\2`, etc.
+
+See: [Git Filter Repo Docs](https://htmlpreview.github.io/?https://github.com/newren/git-filter-repo/blob/docs/html/git-filter-repo.html) for more.
+
+2. Create a clean clone the zed repository, delete tags, delete branches and do the work.
+
+> **Note**
+> If you get `zsh: command not found: #` errors, run:
+> `setopt interactive_comments && echo "setopt interactive_comments" >> ~/.zshrc`
+
+```sh
+rm -rf zed3
+git clone --single-branch --no-tags git@github.com:zed-industries/zed.git zed3
+cd zed3
+
+# This removes the LICENSE symlink
+git filter-repo --invert-paths --path extensions/ruby/LICENSE-APACHE
+
+git filter-repo \
+    --use-mailmap \
+    --subdirectory-filter extensions/ruby/ \
+    --path LICENSE-APACHE \
+    --replace-message ~/projects/expressions.txt
+```
+
+3. Review the commits.
+
+This is your last chance to make any modifications.
+If you don't fix it now, it'll be wrong forever.
+
+For example, a previous commit message was `php/ruby: bump version to 0.0.5`
+which was replaced with `php/bump version to 0.0.5`
+so I added a new line to expressions.txt with `php/==>`
+and next run it became `bump version to 0.0.5`.
+
+4. [Optional] Generate tags
+
+You can always add tags later, but it's a nice touch.
+
+Show you all commits that mention a version number:
+
+```sh
+git log --grep="(\d+\.\d+\.\d+\.)" --perl-regexp --oneline --reverse
+```
+
+Then just:
+```
+git tag v0.0.2 abcd1234
+git tag v0.0.3 deadbeef
+```
+
+Usually the initial extraction didn't mention a version number so you can just do that one manually.
+
+4. Push to the new repo
+
+Create a new empty repo on github under the [zed-extensions](https://github.com/zed-extensions) organization.
+
+```
+git remote add origin git@github.com:zed-extensions/ruby
+git push origin main --tags
+```
+
+5. [Optional]