[docs][tune] remove low value tune examples and references to them

[angelinalg]

These examples are out of date or provide low value. Pruning body of examples to improve discovery of high value examples.
cc: @pcmoritz @richardliaw @matthewdeng

• https://docs.ray.io/en/master/tune/examples/dragonfly_example.html
• https://docs.ray.io/en/master/tune/examples/flaml_example.html
• https://docs.ray.io/en/master/tune/examples/nevergrad_example.html
• https://docs.ray.io/en/master/tune/examples/sigopt_example.html
• https://docs.ray.io/en/master/tune/examples/tune-serve-integration-mnist.html

Checks

• [sx] I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[pcmoritz]

Nice, this is great 🥳

[pcmoritz]

@matthewdeng Can you make a follow up PR that removes the dragonfly-opt package from the dependencies (and any extra code / tests we have)?

[matthewdeng]

@pcmoritz sure thing, can I get your thoughts on what the deprecation policy should be for these types of removals?

[pcmoritz]

For things like dragonfly which are not even really maintained upstream any more https://github.com/dragonfly/dragonfly (last substantial development was in 2020), we can just remove it without any deprecation.

[pcmoritz]

In general, if there is very low usage, very low doc views and/or the project seems abandoned, let's just remove it without deprecation. If you are unsure just use your best judgement :)