Fix link to CSS in default HTML output

[captainbarbosa]

This ensures all.css is visible by the HTML file before the base URL is set, and fixes #95.

This was the most minimal change I could think of to get this to work, but another option could be to change the default base-url argument from "/" to just an empty string "" - let me know if that is preferred and I can make that change.

[mattt]

Thanks for looking into this, @captainbarbosa! We're actually in the process of switching away from <base> (#93), so that's probably going to be the long-term fix. Since you have fresh eyes on the situation, do you see any compelling reasons to stick with <base> or have any thoughts about the best alternative approach?

[captainbarbosa]

Oh, I didn't notice that. #93 looks like a better solution. I can close my PR in favor of a more long-term solution.

I'm not a regular web developer, but one additional thing I noticed is by setting <base>, sub-pages don't reference all.css relative to the rest of the project. So for example, the Docs/Index.html can find the CSS file, but Docs/MyClass/Index.html tries to reference all.css and not ../all.css. This issue occurs after applying #95. I think this is already captured in #93 (comment) so I won't open an issue.

I looked at Jazzy, a similar project, to see if they used <base> tags - it doesn't look like they do. I believe the equivalent config setting for that project is --root-url. It appears they are appending as many ../ that are needed to build out the links for each page, so this might be something to keep in mind when addressing #93.

#93 is likely a better solution than this PR, so I'll close this in favor 👍