ref: Rename example/ directory to _examples/

[tonyo]

This is point 1 of #79 (not turning the example folders into modules here).

go help packages tells us:

Directory and file names that begin with "." or "_" are ignored
by the go tool, as are directories named "testdata".

The main motivation is to avoid any issues with unnecessary 3rd party dependencies in the examples, so that e.g. go mod tidy doesn't add new funny things to the go.mod in root.

Regarding point 2 in #79 (adding go.mod/go.sum to every example folder) -- not doing it right now: after some internal discussion, the future of having examples folder in this repos is somewhat unclear.

Things to check before merging:

• Any links from sentry docs that have to be fixed? Yes: fix(go): Fix links to examples in sentry-go repository sentry-docs#6002