Update adr docs based on markdownlint errors (1/3)

If we want to turn on markdownlint linting in this project, we'll have to clear out the errors it found first.

This MR addresses all the docs in doc/adr:

adr/0001-record-architecture-decisions.md:19:140 MD047/single-trailing-newline Files should end with a single newline character
adr/0002-use-rh-codebase.md:21:104 MD047/single-trailing-newline Files should end with a single newline character
adr/0003-use-operator-sdk.md:22:84 MD047/single-trailing-newline Files should end with a single newline character
adr/0004-integration-of-the-gitlab-chart.md:26:170 MD047/single-trailing-newline Files should end with a single newline character
adr/0005-versioning-of-the-operator.md:23:28 MD034/no-bare-urls Bare URL used [Context: "https://gitlab.com/gitlab-org/..."]
adr/0005-versioning-of-the-operator.md:25:3 MD034/no-bare-urls Bare URL used [Context: "https://gitlab.com/gitlab-org/..."]
adr/0005-versioning-of-the-operator.md:27:10 MD034/no-bare-urls Bare URL used [Context: "https://gitlab.com/gitlab-org/..."]
adr/0005-versioning-of-the-operator.md:36:37 MD034/no-bare-urls Bare URL used [Context: "https://gitlab.com/gitlab-org/..."]
adr/0005-versioning-of-the-operator.md:38 MD012/no-multiple-blanks Multiple consecutive blank lines [Expected: 1; Actual: 2]
adr/0006-gitlab-application-versions-supported.md:13:67 MD044/proper-names Proper names should have the correct capitalization [Expected: GitLab; Actual: Gitlab]
adr/0006-gitlab-application-versions-supported.md:26 MD022/blanks-around-headings/blanks-around-headers Headings should be surrounded by blank lines [Expected: 1; Actual: 0; Above] [Context: "## Consequences"]
adr/0006-gitlab-application-versions-supported.md:30:75 MD047/single-trailing-newline Files should end with a single newline character
adr/0007-dependency-on-other-external-components.md:23:58 MD047/single-trailing-newline Files should end with a single newline character
adr/0008-operator-scoped-to-namespace.md:16:28 MD034/no-bare-urls Bare URL used [Context: "https://gitlab.com/gitlab-org/..."]
adr/0008-operator-scoped-to-namespace.md:32:51 MD034/no-bare-urls Bare URL used [Context: "https://gitlab.com/gitlab-org/..."]
adr/0010-ci-environments.md:32 MD012/no-multiple-blanks Multiple consecutive blank lines [Expected: 1; Actual: 2]
adr/0011-operator-release-process.md:20:54 MD044/proper-names Proper names should have the correct capitalization [Expected: GitLab; Actual: gitlab]
adr/0011-operator-release-process.md:24:32 MD044/proper-names Proper names should have the correct capitalization [Expected: Git; Actual: git]
adr/0011-operator-release-process.md:47:17 MD044/proper-names Proper names should have the correct capitalization [Expected: Git; Actual: git]
adr/0011-operator-release-process.md:65:1 MD004/ul-style Unordered list style [Expected: dash; Actual: asterisk]
adr/0011-operator-release-process.md:71:41 MD044/proper-names Proper names should have the correct capitalization [Expected: Git; Actual: git]
adr/0012-makefile-modifications.md:20 MD012/no-multiple-blanks Multiple consecutive blank lines [Expected: 1; Actual: 2]
adr/0012-makefile-modifications.md:23:17 MD044/proper-names Proper names should have the correct capitalization [Expected: Git; Actual: git]
adr/0013-stable-branch-policy.md:12:71 MD044/proper-names Proper names should have the correct capitalization [Expected: Git; Actual: git]
adr/0013-stable-branch-policy.md:24:5 MD044/proper-names Proper names should have the correct capitalization [Expected: Git; Actual: git]

Related to #363 (closed)