I respect your opinion, but OP has indicated that he wants a professional site...
Actually he described the website as simple but with professional looking responsive design. Although this can be interpreted in different ways, from what I understand, he probably wants to make a simple landing page with information about his startup.
... What I want is a simple but professional looking responsive design with simple navigation.
...there are actually many ways without dealing with PHP, but whatever it is, programming is not that easy, known that </HTML> and CSS; is trivial, but when you want a "professional site" It turns out completely the opposite. Since OP wants a site for his project (not a personal site), He has said himself that he knows the basics of HTML and CSS which is not enough for "Pro website", then of course it is best to hire an expert, because this is a work, not a play.
Once again, you do not need to know PHP or any other server-side programming language to build a professional looking website. And no one can convince me otherwise, since I have been in the website development business for over 25 years. This is not about dynamically generated content, but rather front-end design.
So, if you're still not convinced of the power of HTML5 and CSS3, take a look at some of the examples from awards sites like awwwards:
https://www.awwwards.com/websites/css3/