feat: add series roadmap and navigation feature

[Rat-S]

Description

This PR introduces a series navigation and roadmap feature for posts that belong to a series.

It adds:

• partials/series_nav.html: Provides "Previous Part" and "Next Part" navigation links dynamically based on the current post's index within the series.
• partials/series_roadmap.html: Displays a complete roadmap/index of all posts in the series at the bottom of the article, highlighting the current post.
• Corresponding CSS styling in sass/_site.scss using standard top-level classes.

(Note: This is a re-submission of the previous series PR. All !important tags have been completely removed. The series button styles now peacefully coexist with .inner-post by adding .series-nav-btn to the existing :not() exclusion chain for links, maintaining the codebase's existing styling patterns.)

Motivation

Currently, when authors write a multi-part series, there is no built-in way for readers to easily navigate between the different parts or see the full scope of the series. This addition automatically parses the series and series_order variables in the [extra] front matter to automatically generate this navigation, significantly improving reader experience for long-form content.

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[charlesrocket]

Could you add a live example? You could tag current posts, something like a "media" series that includes Audio, Video, and Image posts.

[Rat-S]

Hi,

Could you add a live example?

If you mean that you want to see how it looks when used, please check

• https://www.deadrat.in/posts/evaporative-cooling-part-1/
• https://www.deadrat.in/posts/the-mechanics-of-cooling/
• https://www.deadrat.in/posts/evaporative-cooling-part-2/

I added it to these posts, but some part of the website is kinda nsfw.

If I misunderstood what you meant, kindly clarify. Thanks.

[charlesrocket]

I like the idea, but the current implementation introduces way too much negative space. Could you place it between the post info and the toc?

[Rat-S]

Hi,
Series nav now appears between reading time and TOC (not after TOC), and I have trimmed the blank space around it.
Thanks.

[charlesrocket]

The roadmap at the bottom looks okay. But the top nav-info still has too much negative space. It should be removed or integrated in ToC (probably no button, and no horizontal separator).