https://imagej.nih.gov/ij/docs/: Mixed-content warning and redirects

Accessing [*Linux Installation*](https://imagej.nih.gov/ij/docs/install/linux.html), the browser warns about mixed-content as the URL for the favicon is not using HTTPS:

PS: To save redirects, on all pages rsb.info.nih.gov could be updated to https://imagej.nih.gov/ij/favicon.ico.

Inspect the Linux Installation page and its HTML source, starting with the favicon link shown in the report. Replace the insecure favicon reference wherever it appears with the HTTPS ImageJ URL, then verify that the page loads without a mixed-content warning or unnecessary redirect.

Written by the indexing model from the issue text.