tutorials: put links in Basic Tutorial 1

Function names and other things in tutorials usually are not links. This makes me hard to read them. So I propose this patch, which places links in Basic Tutorial 1. I converted this tutorial partially. If you find this patch interesting, I will convert rest of this tutorial and some other documentation pages I currently need.

The only way to insert links to code fragments is make code fragments pre-blocks and insert links using a-tags. If you know better solution, please let me know.

Also, I was unable to find good way to link to pages outside of this repository (gst-docs), so I link to them using full URIs, for example:

This must always be your first GStreamer command. Among other things,
[gst_init](https://gstreamer.freedesktop.org/documentation/gstreamer/gst.html?gi-language=c#gst_init)():

-   Initializes all internal structures

Again, if you know better way, let me know