Split examples from Express Core

[UlisesGascon]

This is an initial approach to #5309 (cc: @dougwilson ). All feedback is super welcome. 🙌

What is this about?

The main idea is to remove the /examples and the related tests from this repository and relocate them in https://github.com/expressjs/examples

This PR will remove the /examples/** (a13e252) and /test/acceptance/** (555e726) from this repo and update the documentation regarding the examples to point out the new repository to the users (d9da9ad)

What are the dependencies for this PR?

This PR won't make sense unless we transfer my repository https://github.com/UlisesGascon/express-examples to the express org as https://github.com/expressjs/examples.

The other repo has the same git historial as this repo and the Express core stuff have been removed, so only the examples and the acceptance tests are there. See UlisesGascon/express-examples#1 for more details.

What will happen with the PRs related to examples?

Well, the idea is to port them (as both repos have the same history). As an example I port one commit (UlisesGascon/express-examples@51db5b6 from #5190) in this PR: UlisesGascon/express-examples#3

[UlisesGascon]

I will move this PR to draft again, as I want to include a pipeline to support integration tests again.

[IamLizu]

Perhaps we can avoid integration testing by introducing mutation testing. It would benefit us in two ways,

1. clean repo, no examples in the core
2. strong test suites, making sure the test cases are solid for scenarios when code is changed in a place we don't want and the test doing what it should

ref: expressjs/discussions#269