[Docs] show favicon in documentation

[lisphilar]

What we need to document?

Show favicon in documentation.

[lisphilar]

Reference: https://github.com/tcmetzger/sphinx-favicon

[lisphilar]

1. added sphinx-favicon as a dev dependency
2. add lines to docs/conf.py

# -- General configuration ---------------------------------------------------
extensions = [
    'sphinx-favicon',
]
# -- Extension configuration -------------------------------------------------
favicons = {
    "rel": "icon",
    "sizes": "32x32",
    "href": "https://github.com/lisphilar/covid19-sir/blob/master/docs/logo/covsirphy_favicon.png",
}

Updated with 47ba70d

[lisphilar]

The favicon file (PNG) was saved in this repository and called with the absolute URL.

[lisphilar]

Finally successed with the followings.

html_favicon = "https://raw.githubusercontent.com/lisphilar/covid19-sir/master/docs/logo/covsirphy_favicon.ico"

[lisphilar]

.ico file was required and I created it with free software.