Adding missing write package nav entry (MystenLabs#14571)

## Description 

Nav entry was missing for "write package"

## Test Plan 

Local

---
If your changes are not user-facing and not a breaking change, you can
skip the following section. Otherwise, please indicate what changed, and
then add to the Release Notes section as highlighted during the release
process.

### Type of Change (Check all that apply)

- [ ] protocol change
- [ ] user-visible impact
- [ ] breaking change for a client SDKs
- [ ] breaking change for FNs (FN binary must upgrade)
- [ ] breaking change for validators or node operators (must upgrade
binaries)
- [ ] breaking change for on-chain data layout
- [ ] necessitate either a data wipe or data migration

### Release notes

docs/content/sidebars/guides.js

@@ -39,6 +39,7 @@ const guides = [
 					id: 'guides/developer/first-app',
 				},
 				items: [
+					'guides/developer/first-app/write-package',
 					'guides/developer/first-app/build-test',
 					'guides/developer/first-app/publish',
 					'guides/developer/first-app/debug',