docs: Improves the frontend documentation on announcements

[xfiderek]

Summary of Changes

• Added default value for announcements.enabled to custom config to increase verbosity
• Fixed wrong object name (typo) in the documentation
• Added information about config-default.ts file to docs
• Added information about announcement_client.md README

Tests

I've checked that announcements are disabled when the variable added to custom-config.ts is set to false and that setting it to true enables the feature. No new automated tests are needed IMO.

Documentation

Described above

CheckList

Make sure you have checked all steps below to ensure a timely review.

• PR title addresses the issue accurately and concisely. Example: "Updates the version of Flask to v1.0.2"
  • In case you are adding a dependency, check if the license complies with the ASF 3rd Party License Policy.
• PR includes a summary of changes.
• PR adds unit tests, updates existing unit tests, OR documents why no test additions or modifications are needed.
• In case of new functionality, my PR adds documentation that describes how to use it.

[boring-cyborg]

Congratulations on your first Pull Request and welcome to Amundsen community! If you have any issues or are unsure about any anything please check our Contribution Guide (https://github.com/amundsen-io/amundsen/blob/main/CONTRIBUTING.md)

[Golodhros]

LGTM

[xfiderek]

@Golodhros could you merge it or do we need a second approval? I have insufficient permissions to merge.

[boring-cyborg]

Awesome work, congrats on your first merged pull request!