Ensure consistent page titles and website navigation

[robertoostenveld]

@contsili noticed that some page titles are not consistent. They are actually defined at three places: in the markdown header as title, as the first line starting with # in each file, and in the overview pages for the tutorials, faq and examples.

[contsili]

I will find all the overview pages and I will make all the titles consistent based on the "title:..." section between the three dashed lines in every .md file.

[contsili]

FYI I downloaded Ruby, Bundler, Jekyll in my local Windows machine. The only gem that did not install was wdm from Gemfile. Maybe we can comment out this line. See also other people that had the same issue: https://talk.jekyllrb.com/t/newbie-problems-with-wdm-errors/9233/2

This is fixed now with #811.

[robertoostenveld]

with 1c4d8ea I have added a category to the front matter (i.e., the section between the --- at the start of the markdown pages).

Implemented categories so far are faq, example, tutorial and getting_started.

The "seealso" include can now also filter on the category, as well as on the tags.

[robertoostenveld]

The candling of the category front matter is now similar to that of tags, there are category overview pages, and the category shows below the ToC on the right.

I believe this better distinguishes between the relevance of the documentation format (tutorial/example/faq/getting_started, now indicated as the category) and the documentation content (indicated in the tags).

Note that this is still a try-out and we could easily revert it if we don't like it. However, we could also consider to take it further and add more dimensions along which the pages can be grouped. For example: which dataset is used in the specific page. These datasets are now listed in the tags (for example "MEG-language") and their overview page is here.

Also please note that the category is a singular item, not a list like the tags.