Improve series functionality

Pull Request type

Please check the type of change your PR introduces:

Issue Number(s): #430

The PR consists of 3 commits:

Correction of the series entries in the example site; series is a single value, not an array.
Changes at the code. The entries in params.toml by the original author of the series functionality
```
# showRelatedInArticle = false
# showRelatedInSidebar = false
```
suggest that the default value of this variables is false. The code should behave accordingly. Then I thought it could be useful if both variables can be overridden in a post.
Posts in series sorted by date ascending. In my opinion the articles in series build a sequence and are intended to be read in chronological order, oldest first.

[x] Bug-fixes and new features: I have tested locally with the latest release of Hugo extended. This requirement is a standard.
[x] Bug-fixes, new features, and doc changes: I have updated the relevant documentation as part of this PR.
[x] All PRs: I have signed off (using git commit -s ...), or if not possible due to developer environment constraints, will comment below confirming that I am adhering to the Developer Certificate of Origin.