Add an automatically generated table of contents

[Mateusz-Dobrzynski]

Acceptance Criteria

• A table of contents consisting of anchor links to all headings is generated in every article
• The table of contents graphically represents the document structure, i.e. smaller text / smaller font weight / indentations are used to represent headings lower in the hierarchy
• On desktop, the table of contents is placed to the right of the article content. The content itself is moved to the left
• On mobile, the table of contents is hidden by default and can be accessed with a button under the navigation bar

Remarks
The markdown headings can be linked to without any additional MDX structure, meaning that the solution should create tables of contents also in the already existing articles (https://docs.astro.build/pl/guides/markdown-content/#heading-ids)