Improve GDScript format strings tutorial

[tetrapod00]

• Remove usages of "integral". The adjective "integral" is pretty much only used on this page. While "integral number" is technically more correct, "integer" is a much more commonly understood word. The rest of the docs already uses "integer number" or "integer value" for similar cases.
• Add section on string concatenation. Since this page already compares two ways of modifying strings, this one should be mentioned too.
• Improve organization. Information about the format() function was interleaved in a confusing way. I extracted it out to its own section. The introduction to format strings was also simplified.
• Fix incorrect examples. A couple examples in the format() example table did not match their listed output.

Arguably this page should also be renamed, but I think that will break things?

[mhilbrunner]

Thanks! Merged.

[mhilbrunner]

Cherrypicked to 4.3 in #10346.