feat(docs): batch 10 of proofread/edited docs

[vwheeler63]

• color.rst [link]
• draw.rst [link]
• index.rst (under /docs/details/widgets/) [link]

draw.rst is another big one that required quite a bit of diggin in the C code to sort out what some things meant.

index.rst refers to spangroup which changed names and somehow got out of sync with what batches of proofread/edited documents are pending approval and being committed. So I changed it again here so that the link to spangroup.rst would be intact for the Widgets overview page.

cc: @kisvegabor

Notes

• Update the Documentation if needed. Done.
• Add Examples if relevant. n/a
• Add Tests if applicable. n/a
• If you added new options to lv_conf_template.h run lv_conf_internal_gen.py and update Kconfig. n/a
• Run scripts/code-format.py (astyle v3.4.12 needs to installed by running cd scripts; ./install_astyle.sh) and follow the Code Conventions. n/a
• Mark the Pull request as Draft while you are working on the first version, and mark is as Ready when it's ready for review. Done.
• When changes were requested, re-request review to notify the maintainers. n/a
• Help us to review this Pull Request! Anyone can approve or request changes.

[vwheeler63]

cc: @kdschlosser I thought you might like to see the draw.rst doc since you've been talking about the drawing interface recently. It would seem an "introduction" to that topic, and at some point this might branch out into more advanced use of the sub-functions that begin with lv_draw_...(). It also serves as an introduction to adding your own Draw Unit(s), which will undoubtedly be frequent in LVGL's future.