Fix #2189 broken documentation links and wrong default directory name

[jakubmakielkowski]

Description

1. Documentation links might be mismatched
Links in this repository navigate to devtools-next documentation page instead of devtools-v6. These websites have different /docs structure, resulting in 404 errors.
Changing devtools.vuejs.org to devtools-v6.vuejs.org resolves the problem.

Correct links:

devtools-v6 docs: https://devtools-v6.vuejs.org
devtools-next docs: https://devtools.vuejs.org

List of correct fixed links:

• https://devtools-v6.vuejs.org/
• https://devtools-v6.vuejs.org/guide/faq.html#the-vue-devtools-don-t-show-up
• https://devtools-v6.vuejs.org/guide/devtools-perf.html
• https://devtools-v6.vuejs.org/guide/faq.html
• https://devtools-v6.vuejs.org/guide/open-in-editor.html
• https://devtools-v6.vuejs.org/plugin/plugins-guide.html
• https://devtools-v6.vuejs.org/guide/installation.html
• https://devtools-v6.vuejs.org/guide/installation.html#legacy
• https://devtools-v6.vuejs.org/guide/contributing.html
• https://devtools-v6.vuejs.org/guide/contributing.html#pull-request-guidelines

2. Installation default directory name
The default directory name when cloning the repo is devtools-v6 instead of devtools.
packages\docs\src\guide\contributing.md

Additional context

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).

[jakubmakielkowski]

Resolves #2188