[QuasarAppCI]

What

Updates the README and brings the GitHub Pages site in sync with the current state of the library.

README.md

• Fix minigmp version: 6.1.2 → 6.3.0 (the bundled mini-gmp was updated to GMP 6.3.0 in Update mini-gmp to GMP 6.3.0 (Issue #32) #33, tag v1.1.0)
• Add a Documentation section linking to the GitHub Pages site (https://quasarapp.github.io/QtBigInt/)
• Add a License section (LGPL-3.0)
• Typo fixes: "Power by" → "Powered by", "incuding" → "including", "do not required" → "does not require", "makes as a static library" → "is built as a static library"

pages/index.html (GitHub Pages landing page)

• minigmp 6.1.2 → 6.3.0
• Remove qmake references (qmake build files were deleted in Remove qmake #35)
• Fix the broken link to the removed #build anchor → #include-cmake
• Add explicit README and API Reference links (the README is rendered as the Doxygen main page at doc/index.html and deployed to GitHub Pages by the existing deploy-docs.yml workflow on merge)

Verification

• CMake build (Ninja, Release, Qt 6.12.0): clean
• Test suite tests/QtBigintTest: 26/26 PASS
• Doxygen run: 0 errors; updated README (6.3.0, Documentation + License sections) rendered as the site main page

Closes #36

[QuasarAppCI]

Added the human-readable Doxygen documentation and the usage example requested in the review:

• src/bigint.h: full Doxygen documentation for the whole BigInt class - class overview (arbitrary precision, operator set, division semantics, thread-safety warning), a short inline @code example, an @snippet of the complete example, and brief + detailed docs for every public member. BigInt::log(int base) is documented explicitly as not implemented (it is declared but has no implementation).
• examples/bigint_usage_example.cpp: new standalone-compilable usage example (construction from int/string, arithmetic, pow, powm, gcd, isPrime, comparisons, base-2/10/16 string conversions). Verified: compiles with the command in its header and runs correctly.
• doxygen.conf: EXAMPLE_PATH = examples (renders the standalone example page + the new Examples tab), IMAGE_PATH fixed from the broken ./QtBigInt/res to res, and 14 obsolete tags removed.
• README.md: banner moved below the one-line description so the rendered Doxygen main page has a clean title; link to the example file.

Verification: Doxygen 1.9.8 run exits 0 with 0 errors and 0 warnings; docs/classBigInt.html contains the usage snippet, the standalone page docs/examples_2bigint_usage_example_8cpp-example.html is generated with the Examples tab in the menu, and the banner image from res/ renders on the main page. CMake Ninja Release rebuild OK and QtBigintTest passes 26/26.