Rename AirGradient Nitrogen and Total VOC Indexes to NOx and VOC Index

[MallocArray]

The problem

The sensors from the SPG41 sensor have names that do not match the manufacturer and may confuse end users.

"Nitrogen index" doesn't really represent what is being sensed, as the datasheets consistently call it NOx which means nitrogen oxides and not just Nitrogen. To be the most accurate, and consistent with the manufacturer, I believe renaming it to "NOx Index" is the most clear

"Total VOC index" is not necessarily incorrect, but the datasheets consistently call it "VOC Index" and if it is important to include the Total part, using the very commonly accepted TVOC acronym would be more familiar, although since the manufacturer calls it VOC Index and not TVOC Index

As a side issue I think capitalizing each unique word gives a more polished/professional look to the integrations, so "Carbon Dioxide" and "PM0.3 Count" would also improve the overall appearance.

I can put together a PR if you would like, but wanted to get feedback before blindly submitting these suggested changes.

Related links:
https://pediaa.com/what-is-the-difference-between-voc-and-tvoc/

https://sensirion.com/media/documents/5FE8673C/61E96F50/[Sensirion_Gas_Sensors_Datasheet_SGP41.pdf](https://sensirion.com/media/documents/5FE8673C/61E96F50/Sensirion_Gas_Sensors_Datasheet_SGP41.pdf)

What version of Home Assistant Core has the issue?

2024.6.0

What was the last working version of Home Assistant Core?

No response

What type of installation are you running?

Home Assistant OS

Integration causing the issue

airgradient

Link to integration documentation on our website

https://www.home-assistant.io/integrations/airgradient

Diagnostics information

No response

Example YAML snippet

No response

Anything in the logs that might be useful for us?

No response

Additional information

No response

[home-assistant]

Hey there @airgradienthq, @joostlek, mind taking a look at this issue as it has been labeled with an integration (airgradient) you are listed as a code owner for? Thanks!

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[joostlek]

Sounds good to me.

About the capitalization, we use sentence case everywhere within Home Assistant (or well, we strive to, so there probably are some old ones that do not comply)

[MallocArray]

Submitted a PR, but not sure if only the display name section should be renamed, or if it would be better to rename the underlying structure as well.

I'm not familiar with the PR process here so much, so if this isn't done correctly, feel free to correct it, or reject it and do it the proper way.