Headings: automatically shift 1 down in content pages

**Short description**: There should always be max. 1 title with H1 level. However, at the level of (some) page source files, the highest level of heading is not the page title (which is in the front matter) but the titles within the page content. When editing the content it feels correct to start with the top level heading, but this will lead to undesired result. Therefore we should:
* automatically shift each & every title down one level (e.g. `#` in about.md becomes `##`) - if possible
* manually check all content such that headings start at the second level

**Location**: site-wide

**Why have this**: this enhances readabilty and (apparently) is important for accessability

**More info**: