Update documentation

[tyler-yankee]

Updates to the docs for each example include the following:

• remove top-level README reference to old scripts/ directory
• make consistent calls to install_prereqs using sudo, and fix 'install_prereqs.sh' -> 'install_prereqs'
• make consistent CMake build directories inside their respective example source tree, rather than some at the top level
• sync with [drake_cmake_installed] Replace broken particles example #355 to remove broken example and any reference to it
• remove unnecessary gtest local install/build in drake_cmake_external as it is included in the repo

This addresses #353.

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This change is 

[tyler-yankee]

Reviewable status: 1 unresolved discussion, LGTM missing from assignee betsymcphail, platform LGTM missing (waiting on @BetsyMcPhail)

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a discussion (no related file):
+@BetsyMcPhail for feature review, please!

[BetsyMcPhail]

Reviewed 6 of 6 files at r1, all commit messages.
Reviewable status: 5 unresolved discussions, LGTM missing from assignee betsymcphail, platform LGTM missing (waiting on @tyler-yankee)

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drake_bazel_external/README.md line 14 at r1 (raw file):

sudo setup/install_prereqs

See discussion in #359. @tyler-yankee what are your thoughts on how to proceed here?

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drake_cmake_external/CMakeLists.txt line 101 at r1 (raw file):

  # Or from a commit (download and use "shasum -a 256 'xxx.tar.gz'" on it to
  # get the URL_HASH.
  # URL https://github.com/RobotLocomotion/drake/archive/962483669d015ee2618585ace2c884598fbadbb5.tar.gz

It's not clear to me why these were updated?

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drake_cmake_installed/README.md line 21 at r1 (raw file):

# 1) A specific version (date-stamped)
# curl -O https://drake-packages.csail.mit.edu/drake/nightly/drake-20240214-jammy.tar.gz

BTW Some of the example lines begin with # and some don't. It seems like it would be more clear for the commands to not start with #?

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README.md line 16 at r1 (raw file):

Scripts are provided for various CI instances in each example's respective subdirectory under `.github/`. The intended purpose of each is described below:

* `github_actions`:  exemplifies how to put a project depending on a Drake installation on GitHub Actions

Previously, github_actions and jenkins were subdirectories under scripts/continuous_integration. Please update this section to more accurately describe the current setup.