Add inpatient, outpatient signals doc (#1534)

* Add inpatient, outpatient signals doc

* Update docs/api/covidcast-signals/chng.md

Co-authored-by: minhkhul <[email protected]>

---------

Co-authored-by: minhkhul <[email protected]>

Author: XuedaShen

docs/api/covidcast-signals/chng.md

@@ -29,6 +29,8 @@ commercial purposes.
 
 | Signal | Description |
 | --- | --- |
+| `7dav_inpatient_covid` | Ratio of inpatient hospitalizations associated with COVID-19, based on Change Healthcare claims data that has been de-identified in accordance with HIPAA privacy regulations, smoothed in time with a trailing 7 day average. This historical signal is no longer updated. <br/> **Earliest date available:** 2020-01-01 |
+| `7dav_outpatient_covid` | Ratio of outpatient doctor visits with confirmed COVID-19, based on Change Healthcare claims data that has been de-identified in accordance with HIPAA privacy regulations, smoothed in time with a trailing 7 day average. This historical signal is no longer updated. <br/> **Earliest date available:** 2020-01-01 |
 | `smoothed_outpatient_covid` | Estimated percentage of outpatient doctor visits with confirmed COVID-19, based on Change Healthcare claims data that has been de-identified in accordance with HIPAA privacy regulations, smoothed in time using a Gaussian linear smoother <br/> **Earliest date available:** 2020-02-01 |
 | `smoothed_adj_outpatient_covid` | Same, but with systematic day-of-week effects removed; see [details below](#day-of-week-adjustment) <br/> **Earliest date available:** 2020-02-01 |
 | `smoothed_outpatient_cli` | Estimated percentage of outpatient doctor visits primarily about COVID-related symptoms, based on Change Healthcare claims data that has been de-identified in accordance with HIPAA privacy regulations, smoothed in time using a Gaussian linear smoother <br/> **Earliest date available:** 2020-02-01 |