fix: headings markup

[jurajkapsz]

Fixes semanically incorrect or missing heading tags.

Before:

After:

Note:

• While maybe only a slight increase in Lighthouse score, a proper heading structure and HTML semantics actually have a far more reaching impact for website adoption and user accessibility.
• There are more pages that could be semantically fixed as well, eg about page, changelog etc.
• There are few more cases that can be fixed a11y-wise, eg having sufficient color constrasts.
• I find the Description component to be troublesome, not sure what was its initial idea, but I would recommend to use common markup, which the component now obfuscates without a benefit.