docs: add and enforce readme files for each manager

Closes #5597

lib/manager/ansible-galaxy/readme.md

+Extracts Ansible Galaxy dependencies from configuration files used by the `ansible-galaxy` CLI tool.

lib/manager/ansible/readme.md

+Supports Docker-type dependency extraction from Ansible configuration files.

lib/manager/buildkite/readme.md

+Used for updating Docker dependencies in Buildkite configuration files.

lib/manager/bundler/readme.md

+The `bundler` manager is used to extract dependencies from `Gemfile` and `Gemfile.lock` files.

lib/manager/cargo/readme.md

+Extracts dependencies from `Cargo.toml` files, and also updates `Cargo.lock` files too if found.

lib/manager/cdnurl/readme.md

-**Important**: This manager isn't aware of subresource integrity (SRI) hashes. It will search/replace any matching url it finds, without consideration for things such as script integrity hashes.
+**Important**: This manager isn't aware of subresource integrity (SRI) hashes. It will search/replace any matching url it finds, without consideration for things such as script integrity hashes. Use the `html` manager instead if you need SRI updating.
 
 To enable this manager, add the matching files to `cdnurl.fileMatch`. For example:
 

lib/manager/circleci/readme.md

+The `circleci` manager extracts both `docker` as well as `orb` datasources from CircleCI config files.

lib/manager/composer/readme.md

+Extracts dependencies from `composer.json` files, and keeps the associated `composer.lock` file updated too.

lib/manager/deps-edn/readme.md

+Extracts Maven dependencies from `deps.edn` files.

lib/manager/docker-compose/readme.md

+Extracts all Docker images from with Docker Compose YAML files.

lib/manager/dockerfile/readme.md

+Extracts all Docker images in a `Dockerfile`.

lib/manager/droneci/readme.md

+Extracts docker-type dependencies from DroneCI config files.

lib/manager/git-submodules/readme.md

+Keeps git submodules updated within a repository.

lib/manager/gitlabci-include/readme.md

+Extracts "includes" dependencies from `gitlab-ci.yml` files.

lib/manager/gitlabci/readme.md

+Extracts Docker dependencies from `gitlab-ci.yml` files.

lib/manager/gradle/readme.md

+Renovate extracts Gradle dependencies by calling a custom Gradle script. The Gradle binary is then used to extract Maven-type dependencies.

lib/manager/helm-requirements/readme.md

-Renovate supports updating Helm Chart references within `requirements.yaml` files. If your Helm charts make use of Aliases then you will need to configure an `aliases` object in your config to tell Renovate where to look for them.
+Renovate supports updating Helm Chart references within `requirements.yaml` files.
+
+If your Helm charts make use of Aliases then you will need to configure an `aliases` object in your config to tell Renovate where to look for them.

lib/manager/helmfile/readme.md

+Checks `helmfile.yml` files and extracts dependencies for the `helm` datasource.

lib/manager/html/readme.md

+The `html` manager extracts scripts and CSS includes pointing to cdnjs. It differs from the `cdnjs` manager in that it detects if an integrity hash is already present and updates it too if found.