Removed self links

The spec has a bunch of links that redirect to themselves. This is
confusing, and it's not much of a link. I assume this is done to point
out that this is where the term is being defined. I modified those
occurrences to be bolded instead. Moreover, they were usually followed
by "id links". If those were referenced elsewhere, a user would be
redirected to the middle of a section. Being redirected to the beginning
of the section is much clearer. Here are the rules I tried to follow:

- Removed all links that redirected to the same section the link was in.
- When the linked text was the term being defined in the section, bolded
it.
- Moved all "id links" to right before the section header they belong
in.
- If a section is very large, I left the id links next to the term
definitions.

Note that the technique of adding id links before every
header is damned ugly. The spec.html maker adds those id's to the
headers directly, and that seems like it should be default for
CommonMark as a whole. Maybe for future revisions? How about a new {ToC}
element that benefits from that?