Replace <font> in user docs with <span style>

[Xenonym]

What is the purpose of this pull request? (put "X" next to an item, remove the rest)

• [X] Documentation update

As suggested in #680 (review).

What is the rationale for this request?
The <font> HTML tag is deprecated and should not be used in our documentation.

What changes did you make? (Give an overview)
I replaced uses of <font color="[colour]"> in our documentation with <span style="color: [colour]">.

Is there anything you'd like reviewers to focus on?
Note that the SVG files for FA icons use a <font> tag as well. While the SVG <font> tag is also deprecated, we probably shouldn't arbitrarily modify upstream assets unless necessary.

[yamgent]

Seems like this PR was forgotten, sorry about that!

Propose a merge commit message. This one should be relatively simple to write (talk about font becoming obsolete with citation to the Mozilla standard, then mention replacing font with span+style equivalent substitute).

[Xenonym]

Proposed commit message:

Replace <font> in user docs with <span style>

There are multiple instances in our user docs where we use <font>
to inline style text. However, <font> has been deprecated [1] and
should not be used in our user docs.

Let's replace uses of <font> with <span style> instead to inline style
text in our user docs.

[1]: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/font#Summary

[yamgent]

Nice 👍.