fix slugify: markdown header sections containing only special chars

[ghost]

Summary

Markdown header sections that contain only specials characters like <<, ->
or even ? (as used for function descriptions in the Gerbil Scheme Reference
Docs) end up empty, breaking scrolling, search and sidebar navigation.

Replacing the empty result with an underscore, similar to what happens with
numbers in the front position, fixes all of these problems.

To see the problem in action, go to https://cons.io/reference/actor.html#message-primitives
and scroll down. It will jump to the top of the page. Furthermore, clicking on links
these links in the sidebar does nothing, same thing with the search field in
the navigation bar.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome (latest stable)
• Firefox (latest stable)
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[flozero]

Hello @DwDv thank's for your pr i know it's been a while a create the pr. Sorry for that.

It's look awesome your explanation. In the header you mean in ## ? right ?

Thank's a lot for your time ! 👍

[ghost]

Yes, exactly, @f3ltron, in our docs we have header sections like these: ### -> and ### send.

These are representing functions within the our Scheme programming language dialect. While send is no problem at all, -> creates the following link: https://cons.io/reference/actor.html# that leads to page scrolling issues when js is enabled as well as the other problems described above.

Thanks a lot for looking into this!

[flozero]

yeah i just look at every single one PR. It took time but the core team will try his best :).

Ok i see

[flozero]

Maybe we should add more test for it i will have a look as soon as possible