fix headlines order for better accessibility and seo.

[nisarhassan12]

Addresses the following from #842

Why this is important:

According to https://www.w3.org/WAI/tutorials/page-structure/headings/

[nisarhassan12]

I have corrected the order of the headings for all the pages of the site including the docs. On the blog, the heading order still needs to be corrected. @ChristinFrohne @JohannesLandgraf Can I change the order of the headings in the blog posts as well? Or do I have to check with the authors of the individual blog posts first? Thanks

[chrifro]

Thanks @nisarhassan12
Looks good. It's fine if you change the headline style without asking the author of each blog post. Your changes won't influence the content, so it's fine.

[nisarhassan12]

@ChristinFrohne I have made the changes to the blog posts as well. Please have a look again when you'll have time. Thanks

[chrifro]

Looks good! Thanks

[nisarhassan12]

Thanks, @ChristinFrohne for the review. I'll merge this after doing a final self-review to see if there aren't any pages that are leftout.