Several changes in install/update/download sections

[Jennifer Hodgdon]

I wanted to let you know that on this issue:
https://www.drupal.org/project/user_guide/issues/2950307
we made a change to several English source files:

	modified:   source/en/extend-module-install.txt
	modified:   source/en/extend-theme-install.txt
	modified:   source/en/guide.txt
	modified:   source/en/install-composer.txt
	new file:   source/en/install-decide.txt
	new file:   source/en/install-manual.txt
	modified:   source/en/install-prepare.txt
	modified:   source/en/install-run.txt
	modified:   source/en/install-tools.txt
	modified:   source/en/security-update-core.txt
	modified:   source/en/security-update-module.txt
	modified:   source/en/security-update-theme.txt

This was a fairly big change:

a) The current version of Drush (Drush 9) does not support doing downloads or updates of Drupal Core, themes, modules, or distributions. So, we removed mentions of those outdated Drush commands from various topics.

b) The Installation chapter was kind of confusing. So, we took some of the information that used to be in the install-prepare.txt topic, and moved it into two new topics:

- install-manual.txt

- install-decide.txt

c) We also moved the install-composer.txt file into the Installation chapter (it used to be in a later chapter). This and item (b) meant that we made a change to the guide.txt file (the file that determines the table of contents and order of topics within each chapter).

Here is the commit for reference:
https://cgit.drupalcode.org/user_guide/commit/?id=91289ba

Suggested actions to respond to this change in each language, for each file, would be either:
a) If the file hasn't been translated yet, copy the English source file to your language directory and commit it.
b) If the file has already been translated, open a new issue to revise the translation.