Remove Extension Manager docs

[jeff-matthews]

Purpose of this pull request

Due to a known issue, the Extension Manager does not work in 2.3.x and is being deprecated in 2.3.6.

This pull request (PR):

• Removes the Extension Manager section from the Software Update Guide (only existed in 2.3)
• Redirected Extension Manager docs to the General CLI Installation topic
• Added the Web Setup Wizard deprecation messaging to the Module Manager section in the Software Update Guide (it was missed when we added it to setup and upgrade sections)

See internal ticket MC-34843 for details and links to staging.

Affected DevDocs pages

• https://devdocs.magento.com/guides/v2.3/comp-mgr/extens-man/extensman-checklist.html
• https://devdocs.magento.com/guides/v2.3/comp-mgr/module-man/compman-checklist.html

whatsnew
Removed the Extension Manager section of the Software Update Guide and redirected to CLI installation.

[jeff-matthews]

I forgot to delete images that we no longer use now that several topics have been removed... I'll have to push that up as a separate commit.

[keharper]

Approved.

Consider retitling "General CLI installation" to something like "Install extensions from the CLI". If I'm redirected to this topic, the current title makes me think I'm in the wrong place.

[jeff-matthews]

What about "Install extensions"? Since you can only install extensions using the CLI for 2.3.x and 2.4.x now, the "from the CLI" bit seems unnecessary.

[jeff-matthews]

running tests

[jeff-matthews]

running tests

[ghost]

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