I saw the author mentioned in another comment that they found themselves peeping inside type declaration files "too often". While I do often use sites generated by the above tools to discover new API's that suit my needs, diving into the actual code using a good decompiler is still my first move, as it is often cheaper than seeking out the documentation online, and it will show me the actual implementation as well. So in my opinion there is no shame in looking inside the declaration files!
The direct relative tool to javadoc/docfx is Typedoc [1], which this tsdocs.dev hosts (a fork of). The benefits tsdocs.dev adds on top of Typedoc are the ability to open any arbitrary npm package (plus some smarts for @types/ package redirects) inside a hosted Typedoc whether or not that package's own docs site includes a Typedoc view or not.
I use (Markdown plugin versions of) Typedoc output in some of my packages' docs sites. It is a handy tool and I know some users appreciate having it built with the other docs. I'm also personally equally likely to jump directly to a declaration file rather than pull up a documentation site, but I appreciate things like Typedoc when I do find them on other projects' documentation sites, and I appreciate tsdocs.dev for giving a way to further do it for arbitrary packages that don't include it, even if in many cases that won't be the first tool I use for the job because I'm comfortable enough directly in declarations files.
(Also, you mention a good decompiler, its important to remember that Typescript declaration files generally aren't decompiler artifacts and may be inaccurate to the code actually running. I'm more likely to trust a package that generates its own Typedocs in their documentation as that implies they keep their declarations up to date and/or write their library in Typescript directly. In other sorts of projects for me sometimes the next jump from the declaration file is to the source files. I've PRed a lot of declaration file fixes over the years.)
https://docs.oracle.com/javase/8/docs/technotes/tools/window...
https://dotnet.github.io/docfx/
I saw the author mentioned in another comment that they found themselves peeping inside type declaration files "too often". While I do often use sites generated by the above tools to discover new API's that suit my needs, diving into the actual code using a good decompiler is still my first move, as it is often cheaper than seeking out the documentation online, and it will show me the actual implementation as well. So in my opinion there is no shame in looking inside the declaration files!