Add Sphinx documentation source path and favicon (#41)

* Add path to documentation in Sphinx configuration

Update the Sphinx configuration to indicate where the documentation
source is located. This ensures buttons created with
`use_edit_page_button` correctly reference the documentation source.

* Add favicon for Sphinx documentation

- Add `favicon.png` to `docs/source/_static/`.
- Update Sphinx configuration to use `favicon.png` as the
  documentation page favicon.

This improves the documentation by displaying a custom favicon in
browser tabs.

Author: pavelacamposp

docs/source/_static/favicon.png

@@ -84,6 +84,7 @@ def parse_attributes_section(self: GoogleDocstring, section: str) -> list[str]:
 # -- Options for HTML output --
 html_theme = "sphinx_book_theme"
 html_title = "Direct Data-Driven MPC"
+html_favicon = "_static/favicon.png"
 html_last_updated_fmt = ""
 html_static_path = ["_static"]
 html_css_files = ["styles.css"]
@@ -95,6 +96,7 @@ def parse_attributes_section(self: GoogleDocstring, section: str) -> list[str]:
 
 html_theme_options = {
     "repository_url": "https://github.com/pavelacamposp/direct-data-driven-mpc",
+    "path_to_docs": "docs/source/",
     "use_repository_button": True,
     "use_edit_page_button": True,
     "use_issues_button": True,