Fix #330: replace changelog.txt with changelog.md in the changelog link on the download page

[auge8472]

This fixes #330. The change is necessary to reflect the change of the filetype of the changelog file in OpenTTD/OpenTTD#12994. I have not found a link to the file known-bugs.txt whose file type was also changed in pull request OpenTTD/OpenTTD#12994.

[JGRennison]

Does this not break all the release download pages from before this change was made?
I was presuming that this would need to be made conditional (which is why I didn't make my own PR for it).

[auge8472]

Does this not break all the release download pages from before this change was made?

At first I wanted to say ‘No, the links in the release messages are correctly referencing the changelog.txt of these releases.’ because I asked for these links in the release blog entries in your issue (#330). But now I realise, that you are talking about the download page itself. The page for nightlies and for future releases need the link to the changelog.md but the page for downloading the current release needs the link to changelog.txt. I don't know if it's possible to request downloads of older releases and if it's possible, how to do it.

I was presuming that this would need to be made conditional (which is why I didn't make my own PR for it).

I think, you are right. But what should be the condition?

I'll have to take this apart from what I see in _layouts/download.html. The version number is part of the variable folder. For stable releases it is openttd-releases/[version-number]. For nightlies it would be openttd-nightlies/[year]. The variable folder is composed of parts page.folder (for openttd-releases or openttd-nightlies) and page.version for the version number or the name of the nightly release in line 14 of downloads.html.

{% assign folder = page.version | prepend: "/" | prepend: page.folder %}

Is that so far correct? Should it be so "stupid" simple to define one last/newest release with a changelog.txt as a "magic number" to switch between txt and md? That does not sound right to me.

[stormcone]

There is a page.date variable, maybe that could be use as whether it is before or after that PR's date.

[auge8472]

Ok, I tried to run the site locally by following the guides for the website to install ruby, jekyll and so on and failed on two machines because of different reasons. I am an absolute beginner in Ruby and Jekyll and am overwhelmed by the error messages. This is someone elses turn.