fix broken links in maintaining.md

reverse-string: add to track

circular-buffer: make classes and methods package private in reference solution

crypto-square: make classes and methods package private in reference solution

nth-prime: make classes and methods package private in reference solution

kindergarten-garden: make classes and methods package private in reference solution

run-length-encoding: make classes and methods package private in reference solution

bowling: improve reference solution

bowling: remove unnecessary exceptions from test method signatures

* book-store: change indent from 2 to 4 spaces

* book-store: Update with @FridaTveit 's improvement

difference-of-squares: add starter implementation

* collatz-conjecture: add start implementation.

* collatz-conjecture: update with @FridaTveit 's suggestion

binary-search-tree: simplify reference implementation

nth-prime: add starter implementation

rotational-cipher: add starter implementation

* kindergarten-garden: add starter implementation

* kindergarten-garden: add constructors to starter

anagram: improve reference solution

allergies: update reference and starter implementations

bob: improve reference solution

book-store: delete .keep

binary-search-tree: remove unused import

Add topics to problem 'binary-search' #938

anagram: replace Arrays.asList with Collections.singletonList #981

* isbn: implement exercise in Java track

* isbn-verifier: Update reference solution.

* isbn-verifier: Add starter implementation.

* Add @FridaTveit 's improvements

* Add some more improvements

* saddle-points: add starter implementation.

* saddle-points: Update parameter name.

beer-song: update reference solution to package-private