feat: update Headroom implementation to support recent Hugo versions

[Agos95]

Purpose

TLDR: Fix #2935

As described in #2935, there is a bug in the syntax to set the header sticky. In particular, this is the buggy code:

https://github.com/wowchemy/wowchemy-hugo-themes/blob/57e374fe08d8bb0eee34b160d479a77dc13b8503/modules/wowchemy/layouts/partials/site_js.html#L131

As for now, to make the header sticky you need to set the parameter

header.on_scroll : false

but it should be:

header:
  on_scroll : false

In addition, it actually checks if sticky.header is set and equal to "disappear", but I think this should be a boolean value.

With this PR the sticky header is set following this logic:

1. by default homepage and book layout have a sticky header and all other pages have a disappearing one
2. use the global configuration in params.yaml to set the options to the whole site:

header:
  sticky: true/false

3. use the specific page option, by looking the front matter:

header:
  sticky: true/false

# or this one for backward compatibility
#header.on_scroll: 

The old "buggy" behaviour is still supported in Page params for backward compatibiloity.

See #2950 (comment) for an updated description of the logic of this PR.

Documentation

The logic described above should be added here in the documentation, alongside with the sticky parameter under header in params.yaml

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[github-actions]

This PR is stale because it has not had any recent activity. The resources of the project maintainers are limited, and so we are asking for your help.

If you feel that the PR is still relevant in the latest release, consider making the PR easier to review and finding developers to help review the PR.

Please be mindful that although we encourage PRs, we cannot expand the scope of the project in every possible direction. There will be requests that don't make the roadmap.

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[rodrigoalcarazdelaosa]

This PR is still relevant, as it’s addressing a bug.

[gcushen]

@Agos95 to merge the PR, please refactor the PR so that it is a fix rather than a feature (renaming and converting the options from string to a boolean 'sticky' option is regarded as a new feature rather than a fix).

After we fix this PR, the community can discuss in a separate issue or on Discord if it makes sense to limit Headroom's options to only support a boolean 'sticky' option and no other Headroom features.

[Agos95]

Ok, I refactored the code as requested, keeping the string type for this option, and using the value "disappear" to have a disappearing header.

As description of this PR, I copy the comment I put in the code.

---

Loading the Headroom js set the header to disappear when scrolling the page down.
If you want the header to disappear, set the option to "disappear"; otherwise (sticky header), set it to any other value (eg. "sticky").
This option can be set in the following ways:

• globally, by adding in your 'params.yaml':

  header:
    on_scroll: "disappear" | "sticky"

• per page, by adding in the page Front Matter:

  header:
    on_scroll: "disappear" | "sticky"

• in order to be compatible with older versions where there was a bug, the header also disappears by setting in the page Front Matter (now deprecated):

  header.on_scroll: "disappear" | "sticky"

If the header is set to disappear, the Headroom js is loaded.

By default, the header is sticky in the homepage and in the book layout, and it disappears when scrolling in all other pages.