Fix hard-coded URLs

(Thanks to @parkerbxyz for listing this in another issue! I'm moving to this new issue to be tracked separately)

As a part of open sourcing this repository, and renaming/moving it, one thing we’ll need to address is all of the hardcoded links to this repository that exist throughout its files:

- [x] [README.md](https://github.com/githubtraining/training-manual/blob/1733bc4727429b8ecaba0e63465427d060749a67/README.md#L11)
- [x] [CONTRIBUTING.md](https://github.com/githubtraining/training-manual/blob/656a973f10bfd029d23a1675da5fdbb3bb6dd524/CONTRIBUTING.md#L3)
- [x] [script/reset-game](https://github.com/githubtraining/training-manual/blob/a4459561ba81a385f72c262287f1b677ade1009c/script/reset-game)
- [x] [script/create-repo](https://github.com/githubtraining/training-manual/blob/a4459561ba81a385f72c262287f1b677ade1009c/script/create-repo)
- [x] [docs/GH4D/index.html](https://github.com/githubtraining/training-manual/blob/1188b9b051ee9ac07fbf117c1cc58842e40e5138/docs/GH4D/index.html#L4)
- [x] [docs/GH4DJA/index.html](https://github.com/githubtraining/training-manual/blob/1188b9b051ee9ac07fbf117c1cc58842e40e5138/docs/GH4DJA/index.html#L4)
- [x] [docs/ja/README.md](https://github.com/githubtraining/training-manual/blob/1188b9b051ee9ac07fbf117c1cc58842e40e5138/docs/ja/README.md#L3)
- [x] [docs/ja/app_Day_1_activities.md](https://github.com/githubtraining/training-manual/blob/57b0cb855d1ac5eb71a2edeec93415e58661a76c/docs/ja/app_Day_1_activities.md#L9)

My biggest concern is the teaching scripts. We’ll need to prepare a pull request with changes to the scripts that can be merged right after the rename to avoid disrupting any customer engagements. I think that goes for the remaining steps in the rename process; we’ll likely need to make the majority (if not all) of the changes in quick succession.