Update "implement an exercise from specification"

[FridaTveit]

When updating the Java track documentation we noticed that the "implement an exercise from specification" documentation was slightly out of date in its description of the exercises object. This has been updated with a link to the exercises.md document which explains in detail what should be in the exercises object (in config.json).

Also, the exercises.md document didn't have a link to the topics list so that has been added.

This should hopefully fix #92.

This is my first time making any changes to this repository so please let me know if I should have done anything differently :)

[rpottsoh]

@FridaTveit said:

This is my first time making any changes to this repository so please let me know if I should have done anything differently :)

Some people might prefer to see two commits, one for each file. I don't know that that is necessary here. Good job. Only some minor fixes to make in each file. :+1:

[rpottsoh]

I've looked over your second commit. All the links work. :+1: Thanks for making these changes.

[rpottsoh]

Squash and merge if you end up doing the merge yourself.

[kytrinyx]

This looks great! Thanks so much for updating it, and thanks @rpottsoh for helping review. ✨