Update repository links in subpackages documentation

[mlewand]

There are multiple instances in each package where repository-related links are placed. These will naturally change after the migration so we need to be sure that they're correct.

Examples are:

• Badges in the main README.md's, like here: https://github.com/mlewand/ckeditor5-monorepo-test/blob/880aaabffa8a5f5affe85553b27ab01f6140530d/packages/ckeditor5-link/README.md#L5-L9
• links in docs/api/*.md files, like here: https://github.com/mlewand/ckeditor5-monorepo-test/blob/880aaabffa8a5f5affe85553b27ab01f6140530d/packages/ckeditor5-link/docs/api/link.md#L23-L30

Also it make sense to run a proper search/regexp over all files, including js files - maybe there are some links that should be replaced too.

Subtask of #6479.

[mlewand]

As for the change I think it make sense just to make i/6537 branches in each package with the changes and make one, sort-of, master pull request? This should make PR status maintenance easier.

[pomek]

Changes are done in all repositories:

• ckeditor/ckeditor5-core@master...i/6537
• ckeditor/ckeditor-cloud-services-core@master...i/6537

[mlewand]

Fixed in ckeditor/ckeditor5-core#220 and with merging i/6537 branch to each other package.