fix(makefile): Add check for pnpm

[fredrb]

When running make develop for the first time, I ran into an error where a install binary was being called.

This happens because I didn't have pnpm installed, resulting in $(PNPM) install resolving to install

$ make develop
install
usage: install [-bCcpSsv] [-B suffix] [-f flags] [-g group] [-m mode]
               [-o owner] file1 file2
       install [-bCcpSsv] [-B suffix] [-f flags] [-g group] [-m mode]
               [-o owner] file1 ... fileN directory
       install -d [-v] [-g group] [-m mode] [-o owner] directory ...
make: *** [/Users/fbittenc/go/src/github.com/monetr/monetr/node_modules] Error 64

I've added a check for pnpm in Makefile to show a proper error message when running make develop without pnpm, resulting in the following message:

$ make develop
Error: pnpm not found.
make: *** [check-pnpm] Error 1

I placed check-pnpm as a dependency of $(NODE_MODULES), which itself is a dependency of other steps. Let me know if you think there is a better step to add this in.

[elliotcourant]

Thank you very much!

[elliotcourant]

Once the build passes I'll merge, no need to worry about the mkdocs step; its trying to use the internal image.