Improve depiction of Latex text in docs

[glatterf42]

This PR simply writes \text{xyz} in Latex math mode in the docs where before it used to be xyz. This should improve the time page and the efficiency page.

How to review

• Build the documentation and look at the changed pages

PR checklist

• Continuous integration checks all ✅

[codecov]

Codecov Report

Merging #700 (356cf2c) into main (6a74435) will not change coverage.
The diff coverage is n/a.

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[glatterf42]

On the time page, please check that the equations in example 5 are correct. What I'm wondering about in particular is why the discount factor for 1020 should not contain any information of the years 1000 to 1010, when in the first part it is explained that the discount should be cumulative. There are two options I can see here: either df_1020 should be based on df_1010, in which case the years should be calculated by (1010-1011) etc and df_1010 itself should appear in the equation. Or it should be cumulative from 1000 to 1020, in which case the equation should be twice as long (i.e., start with (1000-1001)).
I briefly looked into the logic behind these equations in general, the present value formula is new to me, but I didn't have the time yet to check if the calculation is done correctly (i.e., is the discount factor defined as PV/C following the definiton on wikipedia?). However, based on the success of MESSAGEix in general, I assume that is the case.

Also, SimpleHTTPServer seems to have moved to http.server, so I adapted the local-build instructions for the docs.