make dev doesn't generate a manifest.json file

### Steps to reproduce
1. `git clone 'https://github.com/hypothesis/client.git'`
2. `cd client`
3. `make dev`
4. `yarn link`
4. `cd ../browser-extension`
5. `yarn install`
5. `yarn link hypothesis`
6. `make`

### Expected behaviour
The browser extension should build per these guides: [client](https://h.readthedocs.io/projects/client/en/latest/developers/developing/#setting-up-a-client-development-environment) and [extension](https://github.com/hypothesis/browser-extension#development).

### Actual behaviour
The final make command fails with `make: *** No rule to make target 'node_modules/hypothesis/build/manifest.json', needed by 'build/client/build'. Stop.`

### Browser/system information
Ubuntu 20.04

### Additional details
- If you check the client's build directory, you can see that the manifest file is indeed missing. This can be resolved by running `make build`, but then I get a different set of issues (and since I'm not looking to run it in production, I think this is the wrong path to go down).
- The [client guide](https://h.readthedocs.io/projects/client/en/latest/developers/developing/#setting-up-a-client-development-environment) looks like it's out of date (if I run `make` in the client repo, I get the `make help` message), so maybe I'm doing something wrong? Is there up to date documentation?