feat: add demo slides for using beamer lectures

[Benno2000]

I created some demo slides to illustrate how to use beamer lectures with the template.

[tthuem]

I guess we need at least three lecture parts to illustrate how it works (e.g., with includeonlylecture). Also, maketitle should be used for every lecture, ideally also with a different lecture title (e.g., subtitle).

[Benno2000]

I updated the example slides for using lectures with our template:

• The lectureDemo-slides define a \lectureoverviewslide command that can once be manually adjusted to contain all lectures. It then displays the list of all defined lectures on the left side and automatically generates a toc of the current lecture on the right side.
• There are three lectures with two sections each to illustrate how lectures can be used together with sections.
• At the beginning of each lecture, a titleslide and a lecture overview is generated.
• It is illustrated how only single lectures can be included using \includeonlylecture

Regarding the template, there are a few modifications in \AtBeginLecture:

• At the beginning of each lecture, a new \part is being generated in order to reset the sections for the toc.
• The section and frame numbers are reset to 0
• The subtitle is set to the current lecture number and title. This enables generating a new title slide for each lecture that has the overall title as title and the lecture title underneath it as subtitle.

[tthuem]

@Benno2000 The links in the PDF to other lectures do not work. Is it a problem on my end? Does it work on your setup?

[tthuem]

The overview slide still has a weird footer, but we can also fix that later:

[Benno2000]

@tthuem

The links in the PDF to other lectures do not work. Is it a problem on my end? Does it work on your setup?

When I tried it a few days ago, the links were not clickable within VS-Code but in Adobe Acrobat it worked fine. However it that does not work anymore now, too. I'll try to figure out what the issue is here.

The overview slide still has a weird footer, but we can also fix that later

That's right, I'd suggest to fix that in a separate issue. For now, we could just use [plain] for the overview slides, which looks better in my opinion anyway in this case.

[Benno2000]

I fixed the lecture links by using \hyperlink instead of \href. Now they should be clickable.

[tthuem]

@Benno2000 Using [plain] for the overview slides sounds good, have not thought of that yet. Go ahead with the merge then.