Adjustments Needed in "Sequence Transformation" chapter

[presnick]

• change name: Sequence Mutation --> Transforming Sequences [already done by Brad]
• change intro to emphasize two kinds of transformations: mutation and copy-with-changes
  • use the mutation vs. copy-with-changes vocabulary at other strategic points in the chapter
• add material on formatting strings into SequenceMutation/NonmutatingMethodsonStrings, using material from https://runestone.academy/runestone/static/thinkcspy/Strings/StringMethods.html#format-strings and https://106w18.learningpython.today/runestone/static/UMSI106/StringFormatting/Interpolation.html

[bnmnetp]

@laumurph I already did the first two big bullet items. I'll leave it to you to add the formatted strings material.