chore: replace go.work with go.work.example (#3216)

As agreed upon, go.work should be used for local development.

This removes it, replacing it with a go.work.example that people can
use. It adds the two files `go.work` and `go.work.sum` to gitignore

.gitignore

@@ -16,4 +16,6 @@ square/testdata
 **/*.html
 .release-env
 **/*.DS_Store
-vendor
+vendor
+go.work
+go.work.sum

Makefile

@@ -39,8 +39,6 @@ install: go.sum
 
 ## mod: Update all go.mod files.
 mod:
-	@echo "--> Syncing workspaces"
-	@go work sync
 	@echo "--> Updating go.mod"
 	@go mod tidy
 	@echo "--> Updating go.mod in ./test/testground"

README.md

@@ -116,6 +116,8 @@ See <https://docs.celestia.org/category/celestia-app> for more information
 
 This repo attempts to conform to [conventional commits](https://www.conventionalcommits.org/en/v1.0.0/) so PR titles should ideally start with `fix:`, `feat:`, `build:`, `chore:`, `ci:`, `docs:`, `style:`, `refactor:`, `perf:`, or `test:` because this helps with semantic versioning and changelog generation. It is especially important to include an `!` (e.g. `feat!:`) if the PR includes a breaking change.
 
+This repo contains multiple go modules. When using it, rename `go.work.example` to `go.work` and run `go work sync`.
+
 ### Tools
 
 1. Install [golangci-lint](https://golangci-lint.run/welcome/install) 1.57.0