Program.cs

var builder = WebApplication.CreateBuilder(new WebApplicationOptions
{
    WebRootPath = "markdown"
});

var app = builder.Build();

app.Use((HttpContext context, RequestDelegate next) =>
{
    var markdown = new MarkdownMiddleware(next, app.Environment);
    return markdown.Invoke(context);
});

app.Run(context =>
{
    return context.Response.WriteAsync("\nIf you read this, the file does not exist.");
});

app.Run();

public class MarkdownMiddleware
{
    readonly RequestDelegate _next;

    readonly IWebHostEnvironment _env;

    public MarkdownMiddleware(RequestDelegate next, IWebHostEnvironment env)
    {
        _next = next;
        _env = env;
    }

    public async Task Invoke(HttpContext context)
    {
        _env.ContentRootPath = Directory.GetCurrentDirectory();
        _env.WebRootPath = Path.Combine(_env.ContentRootPath, "markdown");

        var requestPath = context.Request.Path;

        //Get default page
        if (requestPath == "/")
        {
            var defaultMd = Path.Combine(_env.WebRootPath, "index.md");
            if (!File.Exists(defaultMd))
            {
                context.Response.StatusCode = 404;
                await context.Response.WriteAsync("File Not Found");
                await _next.Invoke(context);
                //write here for post logic
                return;
            }

            //no more processing. This code is shortcircuited.
            context.Response.ContentType = "text/html";
            await context.Response.WriteAsync(ProduceMarkdown(defaultMd));
            return;
        }

        //Replace the path and remove the beginning \ of the path
        //every request path segment represent a folder within markdown folder, e.g. 
        // /about/us is mapped to markdown\about\us.md File
        // /hello is mapped to markdown\hello.md

        var localPath = requestPath.ToString().Replace('/', '\\').TrimStart(new char[] { '\\' }) + ".md";
        var md = Path.Combine(_env.WebRootPath, localPath);
        if (!File.Exists(md))
        {
            context.Response.StatusCode = 404;
            await context.Response.WriteAsync("File Not Found");
            await _next.Invoke(context);
            //write here for post logic
            return;
        }

        //no more processing. This code is shortcircuited.
        context.Response.ContentType = "text/html";
        await context.Response.WriteAsync(ProduceMarkdown(md));
    }

    string ProduceMarkdown(string path)
    {
        var md = File.ReadAllText(path);

        var res = Markdig.Markdown.ToHtml(md);
        return res;
    }
}