Support anchors in SUMMARY

[mcarton]

While porting a book of mine from GitBook to mdBook, I noticed that anchors are not supported in the SUMMARY file:

For example, given the following summary:

- [Foo](Foo.md)
    - [Subfoo 1](Foo.md#anchor-1)
    - [Subfoo 2](Foo.md#anchor-2)
- [Bar](Bar.md)

The following wrong things are going to happen when running mdbook build:

• Files doc/Foo#anchor-1.md and doc/Foo#anchor-2.md are going to be created.
• The navigation panel will only ever highlight "Subfoo 2" whether you click on "Foo", "Subfoo 1" or "Subfoo 2".
• The navigation arrows don't work: the left arrow can't go back, the right arrow points to the last sub-section.
• All 3 links in the Foo section point to Foo.html, which is empty.

[ehuss]

@Dylan-DPC I think it would be good to have some discussion before merging big changes like this. Some issues:

• There is an existing (albeit broken) PR for this at add support for links with anchors from summary, issue #167 #1128.
• There were a number of concerns raised in Allow linking to sections from summary #167 (comment). I think they should be addressed first.
• This is a pretty fundamental change to how mdbook previously assumed every chapter is a separate file. I suspect it will have large ramifications, and will break external tools.
• This seems to be broken for focus on the side-bar. It seems to always focus the last element.
• This breaks backwards compatibility due to the API changes.

[mcarton]

Don't hesitate to revert this if it causes any issue, however:

This is a pretty fundamental change to how mdbook previously assumed every chapter is a separate file. I suspect it will have large ramifications, and will break external tools.

It shouldn't break any existing book though, because files with anchors are completely broken now.

This seems to be broken for focus on the side-bar. It seems to always focus the last element.

This is incorrect: it always focuses the main section, eg. for my book it will display as

even if you click on the Debug.md#format-with link. This is IMO a good thing. To focus the right sub-section would need CSS or JS magic AFAIK.

[Dylan-DPC-zz]

@ehuss ah my bad didnt see the issue.

[Dylan-DPC-zz]

reverted in #1175