Merge string-interpolation docs into scala3 book

[rockpunk]

This is my first stab at tackling #2813. There are many changed files mainly due to renumbering the metadata, however the overview of the change is as follows:

1. Create a new string-interpolation page in scala 3 book based on Josh's excellent _overviews/core/string-interpolation.md. This is after first-look-at-types and before control-structures.md
2. redirect _overviews/core/string-interpolation.md to overviews/scala3-book/string-interpolation.md
3. Moved the string interpolation stuff out of first-look-at-types.md
4. Moved the multi-line string info in first-look-at-types.md to its own Strings section which describes multiline in more detail and links to new string-interpolation.md
5. Modified the custom interpolator example to give a trivial example so readers could cut/copy working code
6. s/John/James/g just to be consistent

[julienrf]

Thank you for your contribution! Please see my review below.

[rockpunk]

Thanks Julien, will update with suggestions later.

[julienrf]

Thank you very much for your contribution! Do you mind fixing the Git conflicts so that we can merge the PR?

[bishabosha]

removing the link from Guides & Overviews could probably come in a new PR where we make more sweeping changes to the site's navigation

[bishabosha]

thanks @rockpunk for all the work!

[julienrf]

@bishabosha I thought the entry in Guides & Overviews would be removed when we removed the file string-interpolation.md. What should we do instead?