Move or remove conflicting quickstart docs for splice

### Context

Some of the CN quickstart docs merged as part of https://github.com/DACH-NY/docs-website/pull/451 are not fully aligned with how splice, CN apps, and quickstart should be positioned.

Concretely these are:
- intro to splice: https://github.com/DACH-NY/docs-website/pull/451/files#diff-f28a5daf9419af64a3155609673dbaae3b2c486c1107d215e3880f1a81a37819
- upgrades on the global synchronizer: https://github.com/DACH-NY/docs-website/pull/451/files#diff-433c54c59246085cfbfcc9536d1e30f37dbbea96d25324dfb115319c3fd80e04

They are thus not hooked up to the ToC, and tagged as `:orphan:` pages.

### Outstanding work

- [ ] figure out what should be kept
- [ ] create PR for `cn-quickstart` repo
- [ ] update ToC on `docs-website` with the changes

Start by reviewing the two pages linked from PR #451: “intro to splice” and “upgrades on the global synchronizer,” noting why they are orphaned and how they relate to the cn-quickstart and docs-website projects. Determine what should remain, create the requested cn-quickstart PR, and update the docs-website table of contents accordingly.

Written by the indexing model from the issue text.