RawGit has been deprecated and is returning 404 for all SVG assets. Also added bulgarian translation

[bhubavenski]

Fix broken RawGit links + Bulgarian translation 🇧🇬

RawGit has been deprecated and all SVG diagrams in the GitHub Pages version are currently returning 404 errors.
This PR replaces all rawgit.com links with raw.githubusercontent.com, restoring the broken images and clickable diagrams.

Additionally, I have added a full Bulgarian translation of the book.

---

Temporary workaround (until this PR is merged)

For anyone reading the book via GitHub Pages and experiencing broken images, you can run the following script in the browser console. It will automatically fix all RawGit links on the current page:

(function fixRawGitLinks() {
  const RAWGIT = "https://rawgit.com/";
  const RAW_GH = "https://raw.githubusercontent.com/";

  let count = 0;

  document.querySelectorAll("img").forEach(img => {
    if (img.src.includes("rawgit.com")) {
      img.src = img.src.replace(RAWGIT, RAW_GH);
      count++;
    }
  });

  document.querySelectorAll("a").forEach(a => {
    if (a.href.includes("rawgit.com")) {
      a.href = a.href.replace(RAWGIT, RAW_GH);
      count++;
    }
  });

  console.log(`Fixed ${count} rawgit links.`);
})();