tools: simplify HTML generation

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

In tools/doc/html.js we have 3 operations that have little benefit but complicate the file significantly.

1. Asynchronous loading of doc/template.html.
2. Asynchronous loading of doc/api/_toc.md.
3. Requiring tools/doc/preprocess.js and calling its main function.

While doing 1 and 2 asynchronously can have a performance benefit, it is rather small (both files are ~1.5 KB). Besides, the downsides are significant:

1. tools/doc/html.js is required in tools/doc/generate.js and test/doctool/test-doctool-html.js. While the first script calls the main function once per requiring, the second script calls it many times and all these times the same doc/template.html is loaded needlessly.

2. Asynchronous loading of doc/ap/_toc.md makes the script overcomplicated to the extent of this comment:

https://github.com/nodejs/node/blob/2a30bfe295eadbdaea3675447b9fe743c622b47b/tools/doc/html.js#L45

3. The main aim of the tools/doc/preprocess.js is processing @include instructions. Also, as one goes, it strips special comments. There are no @include instructions in doc/ap/_toc.md so the requiring this module is excessiveness.

This PR try to eliminate all downsides in this way:

1. As we have only one HTML template, we can exclude it as an option from cli chain, incorporate its path in tools/doc/html.js and preload it synchronously once per module initialization.

2. We can preload doc/api/_toc.md synchronously and delete a huge async machinery.

3. We can replace tools/doc/preprocess.js requiring with one-liner comment stripping in place.

This PR reduces the code almost by 50 lines producing the same result.

Please, ignore any remaining stylistic issues while reviewing the changes: I am going to modernize and optimize the whole script after this PR. This change is singled out to not mingle logic refactoring with many small changes scattering over the file.