A few spelling fixes, and a few more words added to the spelling
exceptions file.

Removes the value_stream_analytics_create_multiple_value_streams
feature flag

Changes alert boxes to match style guide

Corrects clear typos in doc/ from Vale warnings

Group-level Value Stream Analytics (VSA) was introduced years after
project-level VSA, with documentation that simply pointed to the
original project-level VSA documenation. However, the features have
diverged over time, so now group-level VSA is much more feature-rich.
It deserves its own documentation page now, so this creates it. This
also removes group-level documentation that had been added to the
project-level page.

The previous URL had two drawbacks:

1. It didn't match what the title was about (`Assignments to DevOps
   Stages and Groups`, and the anchor was `#designated-technical-writers`).
2. It didn't account for the other sections, like development and API
   guidelines.

The group formerly known as Manage:Analytics was briefly re-named
Manage:Value Stream Management before being ultimately re-named
Manage:Optimize. This updates the deprecated references to
Value Stream Management in the documentation to be Optimize.

:

This adds a paragraph below the "Days to completion" header on the
group-level VSA page. The intent is to explain what the chart below
represents.

removed confusing mentions of tier where
I believe it was not clear as sections may apply
to both group- and project- level VSA

Correct **CORE** and **PREMIUM** to be encased
within parentheticals so that the markdown parsing
correctly shows them as tier badges.

Previously Value Stream Analytics required that the production
environment be named exactly "production" (lowercase) to be compatible
with the default stages. This relaxes that requirement so "prod" and
"gprd" are allowed, and also so capitalization is irrelevant. This will
make VSA more fully usable by a wider array of customers, and also by
GitLab itself.

This clarifies that Value Stream Analytics exists at the project and
group levels, but the group level feature is EE-only. This also
clarifies that the 7 stages referenced are the default, and can be
customized at the group level.

Make links point directly to the target page,
not the redirect

Total stage is redundant and shown as Lead time
so we don't need it as separate stage anymore.

Adds a delete button for the
currently selected value stream

Ensure we can only delete custom value streams

Fix reminaing specs

* Remove starter-only
* Remove core-only
* Remove premium-only
* Remove ultimate-only
* Remove ultimate
* Remove premium
* Remove starter
* Remove core

Removes the request for the duration
chart median value

Adds changelog entry

Enables the value_stream_analytics_create_multiple_value_streams
feature flag

Removes the value stream analytics filter
bar feature flag and updates specs.

Recently instance-level Value Stream Analytics was deprecated, however
there was still a note in the documentation encouraging users to use
it. This deletes that note.

This adds a section to the Value Stream Analytics documentation to
explain how the lead time and cycle time metrics are calculated.

Links to issues need to be update to use a hyphen,
this MR changes issue links in /doc/user

This change removes the `customizable_cycle_analytics` feature flag and
also some remaining test cases for a previously removed feature flag:
`tasks_by_type_chart`.

- Add a mention od Milestone lists to the terminology section.
- Add links to the Milestone and Assignee lists.
- Style: split long lines.
- Style: add a divider if there's an image followed by a heading.
- Style: don't say "simply".
- Rearrange the Milestone and Assignee lists sections to reflect
  the order in UI.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/196779

Adds additional documentation about the
tasks by type chart to value stream analytics

This commit changes redirected links to their destinations.

Update CI/CD naming in various docs

Previously the documentation for customizable Value Stream Analytics
did not explain that it is only available at the group-level feature,
so this adds a note to the docs.

Update links to avoid redirects, and fix links
with 404 fails