some improvements to [shapes.md]

[Mo-Gul]

Feel free to cherry-pick.

(Other) Remarks:

• Julia is the only (open source) language where no output (i.e. the plots) are shown. Is that intended?

• I can't build the docs locally with Julia 1.11.3. Using the given Manifest.toml I receive various precompilation errors. Updating first to most recent packages and running make html I get various

  Warning: failed when running filled-area-plots.md
  Error is:
  ErrorException("Could not parse yaml frontmatter. Make sure it is separated by ---")

  errors. (Maybe this is related to the previous point.)

• missing links (to other parts of the docs)

  plotlyjs.jl-docs/julia/shapes.md

  Line 256 in 8a874cf

  _Note:_ there are [special methods `add_hline`, `add_vline`, `add_hrect` and `add_vrect` for the common cases of wanting to draw horizontal or vertical lines or rectangles] that are fixed to data coordinates in one axis and absolutely positioned in another.

  plotlyjs.jl-docs/julia/shapes.md

  Line 545 in 8a874cf

  This shape-drawing feature is particularly interesting for annotating graphs, in particular [image traces] or [layout images](/julia/images).

• (at least) the commands add_hline!, add_vline!, add_hrect! and add_vrect! aren't found when I search for them in the manual.
  I'd say at least every command should have a corresponding manual entry and should be findable by the search function.

• Is it intended that 6 values of x|yref are given as chars, e.g. 'x' instead of as strings?

• I suspect that in

  plotlyjs.jl-docs/julia/shapes.md

  Line 447 in 8a874cf

  attr(type="rect", xref="x2", yref="y4",

  the "y4" should be a "y2", right? If this is intended, I suggest to add a comment to the code.