Fix the TOC generation with unique links

[javiereguiluz]

Still WIP. It fixes #155.

I can't find any way of getting the id attributes of headings without changing the vendor dependencies ... so this proposes to just parse the generated HTML file and extract the information from it.

[javiereguiluz]

This is now ready for review.

@wouterj, @alamirault, @OskarStark if you have some time, please help me reviewing this. Thanks a lot!

[javiereguiluz]

After the last changes, I rebuilt Symfony Docs locally and it worked as expected.

The problem was that the static variable that holds header IDs (which we need to be static for it to work) wasn't reset before parsing each document.

In practice, the code made header IDs unique across all files ... and now they are unique within each single file.

[OskarStark]

Looks good to me

[wouterj]

In practice, the code made header IDs unique across all files ... and now they are unique within each single file.

Note that per spec, explicit references (the .. _some-ref: syntax) must be unique across all files whereas implicit references (the one automatically generated based on section titles) must be unique within each single file.

This is something we're tackling in the new parser, but let's keep this parser & library simple and practical.