Add -mod=mod to 'go run' commands in documentation

[llimllib]

https://go.dev/ref/mod#build-commands is the documentation for the -mod flag.
It seems that basically -mod=mod is never harmful in ent's case, all it does is
tell the go command that it can add ent to the go.mod file if it's not already
present.

I removed the recommendation in the "getting started" guide to run go install,
per @masseelch in #2880, but I did not remove it from the README. I think it
might be a good idea to do so, if the rest of the docs are suggesting users
execute the program via go run -mod=mod? But I'll leave that for another PR.

I updated all the documentation sites, but did not update any blog posts - I can
do so if you think that's a good idea. Here are the remaining usages of "go run"
without the -mod flag:

$ rg -tmd "go run ent"
doc/website/blog/2021-11-1-sync-to-external-data-systems-using-hooks.md
37:go run entgo.io/ent/cmd/ent init User

doc/website/blog/2021-09-02-ent-extension-api.md
55://go:generate go run entc.go

doc/website/blog/2022-05-09-versioned-migrations-sum-file.md
97:go run entgo.io/ent/cmd/ent init User

doc/md/tutorial-todo-gql.md
184://go:generate go run entc.go

examples/entcpkg/README.md
57://go:generate go run entc.go

closes #2880

[a8m]

💯

Thanks for adding this @llimllib