Fix #330: Markdown changelog links

[merni-ns]

Closes #330.

Fix the links in download pages and 15.0-beta1 blogpost to point to the new changelog locations.

For 15.0-beta1, this will still point to an empty page since the actually generated changelog is broken (see OpenTTD/OpenTTD#13195).

Is there any other place where changelogs are linked?

[stormcone]

The _layouts/download.html is used not just for OpenTTD itself, but also for other files, like OpenGFX, grfcodec, etc. Those are still using plain text format for change logs. This is currently not a problem, as the latest versions of those are released before the markdown change were done, but can be for the future releases.

[TrueBrain]

The _layouts/download.html is used not just for OpenTTD itself, but also for other files, like OpenGFX, grfcodec, etc. Those are still using plain text format for change logs. This is currently not a problem, as the latest versions of those are released before the markdown change were done, but can be for the future releases.

If I remember correctly, raw_type contains something like openttd-nightlies or openttd-releases, which can tell that it is actually an OpenTTD release. But yeah, adding this additional check would be good, just to avoid future issues :)

[auge8472]

See also #331 for further details. I was unable to solve #330 but maybe there is something interesting to read.