Fix search for occur to account for _n headings

[TotalVerb]

Currently, only the first duplicated heading will have its ID renumbered, because the search for occur only looks for headings with the same ID. All of the duplicate headings after the first will be called heading_2:

### Heading (heading)
### Heading (heading_2)
### Heading (heading_2)

This changes the logic to look for all headings with the same refstring. There is probably still a problem if the user creates a document like

### Heading 2 (heading_2)
### Heading (heading)
### Heading (heading_2)

since the search won't find the explicit heading_2 refstring. I'm not sure how to address this cleanly, however, so I opted for a more minimal change.

[tlienart]

I guess one way to go is to do something a bit like in Documenter where the first occurence gets a _1 and then _2 etc etc so in the pathological situation you suggest:

### Heading 2 (heading_2_1)
### Heading (heading_1)
### Heading (heading_2)

Do you want to give it a shot? otherwise I'm happy to merge your contribution and tackle this later on. Thanks a lot!

[TotalVerb]

Yeah, Documenter.jl always numbers its links. But making that change would break existing section links generated by JuDoc, and in my opinion makes the links less understandable in the common case.

Another option is to reserve some sequence like double underscore __ for numbering, and make refstring collapse double underscore to single underscore, which would probably be a little less disruptive?

[tlienart]

Sounds good. I'll merge this and open an issue with your suggestion for consideration, thanks!