Update text on DORA 4 charts

What does this MR do?

Updates some text on the project-level DORA 4 charts:

Changes to the deployment frequency charts

• The "Deployments" tab has been renamed to "Deployment frequency"

• The title inside this tab ☝ has been updated from "Deployments charts" to "Deployment frequency"

• The description text has been changed from:

  These charts display the frequency of deployments to the production environment, as part of the DORA 4 metrics. The environment must be named production for its data to appear in these charts. Learn more.

  to:

  The chart displays the frequency of deployments to production environment(s) that are based on the deployment_tier value. Learn more.

• The Y axis has been renamed from "Deployments" to "Number of deployments"

• The legend has been renamed from "Deployments" to "Deployment frequency"

• The tooltip text has been updated from "Deployments" to "Number of deployments"

Changes to the lead time charts

• The description text has been changed from:

  These charts display the median time between a merge request being merged and deployed to production, as part of the DORA 4 metrics. Learn more.

  to:

  The chart displays the median time between a merge request being merged and deployed to production environment(s) that are based on the deployment_tier value. Learn more..

• The title inside the tab has been updated from "Lead time charts" to "Lead time"

Changes to documentation

This MR also makes a few small documentation updates.

Screenshots

Description	Before	After
Deployment frequency charts
Lead time charts

Related to #327606 (closed)