Update docs for building Zed (#8092)

Marshall Bowers created 

This PR updates the docs for building Zed to fix the links in the
sidebar after the addition of the Linux-specific docs in #8083.

Release Notes:

- N/A

Change summary

docs/src/SUMMARY.md                            | 2 ++
docs/src/developing_zed__building_zed.md       | 6 ++++++
docs/src/developing_zed__building_zed_linux.md | 2 +-
docs/src/developing_zed__building_zed_macos.md | 2 +-
4 files changed, 10 insertions(+), 2 deletions(-)

Detailed changes

docs/src/SUMMARY.md 🔗 

@@ -19,6 +19,8 @@
 
 - [How to Contribute]()
 - [Building from Source](./developing_zed__building_zed.md)
+  - [macOS](./developing_zed__building_zed_macos.md)
+  - [Linux](./developing_zed__building_zed_linux.md)
 - [Local Collaboration](./developing_zed__local_collaboration.md)
 - [Adding Languages](./developing_zed__adding_languages.md)
 - [Adding UI]()

docs/src/developing_zed__building_zed.md 🔗 

@@ -0,0 +1,6 @@
+# Building from Source
+
+See the platform-specific instructions for building Zed from source:
+
+- [macOS](./developing_zed__building_zed_macos.md)
+- [Linux](./developing_zed__building_zed_linux.md)