examples/readme: fix SAUL spelling mistake
examples/readme: Link to README.md instead of folder
examples/readme: increase title size
examples/readme: make networking category name a bit clearer
examples/readme: move nimble to own category within BLE
examples/readme: move dtls to own category
examples/readme: move heart rate sensor to nimble category
examples/readme: move arduino sketch to languages
examples/timer_periodic: fix file name issue
examples/readme: change level wording about nanocoap
