two-fer: add to track

Minesweeper simplify

food-chain: add to track

transpose: add to track

Fix link to contributing guide

The link was pointing to a not very helpful page in the x-api repo.
It's now pointing to the contributing guide in the docs repo.

kindergarten-garden: clarify test names

For the custom garden tests it wasn't very clear that the students will
be sorted in alphabetial order before being assinged their plants. This
has been added to the test names to make it more clear.

MAINTAINING.md: add link to docs repo

Remove reference to project board

We haven't been using it to set priorities for a while now.

Remove link to dead todo repository

Replace references to repo name `xjava` with references to repo name `java`

all-your-base: update to v1.1.0 of tests

Remove leading 'x' from repository name