Minor clarifying changes to installation docs

[smokestacklightnin]

:pencil: Description

Type: :memo: documentation

This pull request adds a note in the installation docs about activating the virtual environment before installing stardis

There are also very minor changes to the non-developers install instructions (original).

:vertical_traffic_light: Testing

How did you test these changes?

• [X] Testing pipeline

:ballot_box_with_check: Checklist

• [x] I requested two reviewers for this pull request
• [X] I updated the documentation according to my changes

[codecov[bot]]

Codecov Report

Merging #109 (21d535b) into main (1d9daf7) will decrease coverage by 0.94%. Report is 2 commits behind head on main. The diff coverage is 16.12%.

:exclamation: Current head 21d535b differs from pull request most recent head 3a946da. Consider uploading reports for the commit 3a946da to get more accurate results

@@            Coverage Diff             @@
##             main     #109      +/-   ##
==========================================
- Coverage   34.97%   34.04%   -0.94%     
==========================================
  Files          17       17              
  Lines         629      658      +29     
==========================================
+ Hits          220      224       +4     
- Misses        409      434      +25     

Files Changed	Coverage Δ
stardis/base.py	37.50% <ø> (ø)
stardis/model/base.py	11.36% <0.00%> (-2.28%)	:arrow_down:
stardis/model/composition/base.py	50.00% <ø> (ø)
stardis/io/model/marcs.py	68.42% <17.24%> (-22.49%)	:arrow_down:

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[tardis-bot]

*beep* *bop* Hi human, I ran benchmarks as you asked comparing main (1d9daf7e956c4cec0f3ffc8ac3db73d545266b92) and the latest commit (3a946dab7fba0de1e8ffe26594d9c7c553b988b9). Here are the logs produced by ASV. Results can also be downloaded as artifacts here. Significantly changed benchmarks:

```diff ```

All benchmarks:

```diff All benchmarks: before after ratio [1d9daf7e] [3a946dab] 15.1±0.03s 15.1±0.02s 1.00 run_stardis.BenchmarkRunStardis.time_run_stardis ```