docs: improve documentation for GitLab datasources (#19393)

Co-authored-by: HonkingGoose <34918129+HonkingGoose@users.noreply.github.com>

lib/modules/datasource/gitlab-packages/readme.md

@@ -4,6 +4,9 @@ To specify which specific repository should be queried when looking up a package
 
 As an example, `gitlab-org/ci-cd/package-stage/feature-testing/new-packages-list:@gitlab-org/nk-js` would look for the`@gitlab-org/nk-js` packages in the generic packages repository of the `gitlab-org/ci-cd/package-stage/feature-testing/new-packages-list` project.
 
+To specify where to find a self-hosted GitLab instance, specify `registryUrl`.
+An example would be `https://gitlab.company.com`.
+
 If you are using a self-hosted GitLab instance, please note the following requirements:
 
 - If you are on the `Free` edition, this datasource requires at least GitLab 13.3

lib/modules/datasource/gitlab-releases/readme.md

+[GitLab Releases API](https://docs.gitlab.com/ee/api/releases/) supports looking up [releases supported by GitLab](https://docs.gitlab.com/ee/user/project/releases/) and can be used in combination with [regex managers](https://docs.renovatebot.com/modules/manager/regex/) to keep dependencies up-to-date which are not specifically supported by Renovate.
+
+To specify which specific repository should be queried when looking up a package, the `depName` should be set to the project path.
+
+As an example, `gitlab-org/ci-cd/package-stage/feature-testing/new-packages-list` would look for releases in the `gitlab-org/ci-cd/package-stage/feature-testing/new-packages-list` project.
+
+To specify where to find a self-hosted GitLab instance, specify `registryUrl`.
+An example would be `https://gitlab.company.com`.
+
+Please note the following requirements:
+
+- This datasource requires at least GitLab 11.7
+
+**Usage Example**
+
+A real-world example for this specific datasource would be maintaining package versions in a config file.
+This can be achieved by configuring a generic regex manager in `renovate.json` for files named `versions.ini`:
+
+```json
+{
+  "regexManagers": [
+    {
+      "fileMatch": ["^versions.ini$"],
+      "matchStrings": [
+        "# renovate: datasource=(?<datasource>.*?) depName=(?<depName>.*?)( versioning=(?<versioning>.*?))?( registryUrl=(?<registryUrl>.*?))?\\s.*?_VERSION=(?<currentValue>.*)\\s"
+      ],
+      "versioningTemplate": "{{#if versioning}}{{{versioning}}}{{else}}semver{{/if}}"
+    }
+  ]
+}
+```
+
+Now you may use comments in your `versions.ini` files to automatically update dependencies, which could look like this:
+
+```ini
+# renovate: datasource=gitlab-releases depName=gitlab-org/ci-cd/package-stage/feature-testing/new-packages-list versioning=semver registryUrl=https://gitlab.com
+NKJS_VERSION=3.4.0
+```
+
+By default, `gitlab-releases` uses the `semver` versioning scheme.

lib/modules/datasource/gitlab-tags/readme.md

+[GitLab Tags API](https://docs.gitlab.com/ee/api/tags.html) supports looking up [Git tags](https://docs.gitlab.com/ee/topics/git/tags.html#tags) and can be used in combination with [regex managers](https://docs.renovatebot.com/modules/manager/regex/) to keep dependencies up-to-date which are not specifically supported by Renovate.
+
+To specify which specific repository should be queried when looking up a package, the `depName` should be set to the project path.
+
+As an example, `gitlab-org/ci-cd/package-stage/feature-testing/new-packages-list` would look for releases in the `gitlab-org/ci-cd/package-stage/feature-testing/new-packages-list` project.
+
+To specify where to find a self-hosted GitLab instance, specify `registryUrl`. An example would be `https://gitlab.company.com`.
+
+**Usage Example**
+
+A real-world example for this specific datasource would be maintaining package versions in a config file.
+This can be achieved by configuring a generic regex manager in `renovate.json` for files named `versions.ini`:
+
+```json
+{
+  "regexManagers": [
+    {
+      "fileMatch": ["^versions.ini$"],
+      "matchStrings": [
+        "# renovate: datasource=(?<datasource>.*?) depName=(?<depName>.*?)( versioning=(?<versioning>.*?))?( registryUrl=(?<registryUrl>.*?))?\\s.*?_VERSION=(?<currentValue>.*)\\s"
+      ],
+      "versioningTemplate": "{{#if versioning}}{{{versioning}}}{{else}}semver{{/if}}"
+    }
+  ]
+}
+```
+
+Now you may use comments in your `versions.ini` files to automatically update dependencies, which could look like this:
+
+```ini
+# renovate: datasource=gitlab-tags depName=gitlab-org/ci-cd/package-stage/feature-testing/new-packages-list versioning=semver registryUrl=https://gitlab.com
+NKJS_VERSION=3.4.0
+```
+
+By default, `gitlab-tags` uses the `semver` versioning scheme.