Update footer to report current year without change every file

[masesdevelopers]

Is your feature request related to a problem? Please describe.
Currently when the year changes more or less all files shall be updated.
Footer is a common pattern and it is common for the website, but docfx appends it to every file: this behavior imposes to update all files just to change a single number and the sitemap will be regenerated at all, changing/losting all lastmod time references.

Describe the solution you'd like
Probably there are two solution:

1. put the footer in an external file and create an "include" mechanism from each page
2. substitute the year with an auto-generated value from current date

Describe alternatives you've considered
N/A

Additional context
N/A

[masesdevelopers]

Modification done with #39 successfully verified with #40.