git-merge-file: add page

[MQ-06]

• [ ] The page(s) are in the correct platform directories: common, linux, osx, windows, sunos, android, etc.
• [ ] The page(s) have at most 8 examples.
• [ ] The page description(s) have links to documentation or a homepage.
• [ ] The page(s) follow the content guidelines.
• [ ] The page(s) follow the style guide.
• [ ] The PR title conforms to the recommended templates.
• Version of the command being documented (if known):

[CLAassistant]

All committers have signed the CLA.

[tldr-bot]

The build for this PR failed with the following error(s):

pages/common/git-merge-file.md:10 MD001/heading-increment Heading levels should only increment by one level at a time [Expected: h2; Actual: h3]
pages/common/git-merge-file.md:13 MD031/blanks-around-fences Fenced code blocks should be surrounded by blank lines [Context: "```bash"]
pages/common/git-merge-file.md:18 MD031/blanks-around-fences Fenced code blocks should be surrounded by blank lines [Context: "```bash"]
pages/common/git-merge-file.md:23 MD031/blanks-around-fences Fenced code blocks should be surrounded by blank lines [Context: "```bash"]
pages/common/git-merge-file.md:28 MD031/blanks-around-fences Fenced code blocks should be surrounded by blank lines [Context: "```bash"]
pages/common/git-merge-file.md:33 MD031/blanks-around-fences Fenced code blocks should be surrounded by blank lines [Context: "```bash"]
pages/common/git-merge-file.md:38 MD031/blanks-around-fences Fenced code blocks should be surrounded by blank lines [Context: "```bash"]
pages/common/git-merge-file.md:43 MD031/blanks-around-fences Fenced code blocks should be surrounded by blank lines [Context: "```bash"]
pages/common/git-merge-file.md:48 MD031/blanks-around-fences Fenced code blocks should be surrounded by blank lines [Context: "```bash"]
pages/common/git-merge-file.md:55 MD031/blanks-around-fences Fenced code blocks should be surrounded by blank lines [Context: "```bash"]
pages/common/git-merge-file.md:60 MD031/blanks-around-fences Fenced code blocks should be surrounded by blank lines [Context: "```bash"]
pages/common/git-merge-file.md:65 MD031/blanks-around-fences Fenced code blocks should be surrounded by blank lines [Context: "```bash"]
pages/common/git-merge-file.md:
Error: Parse error on line 4:
# git-merge-file```bashgit merge-fi
----------------^
Expecting 'TEXT', 'GREATER_THAN', got 'BACKTICK'
pages/common/git-merge-file.md:1: TLDR001 File should contain no leading whitespace

Please fix the error(s) and push again.

[spageektti]

Hi. Thanks for your contribution. Have you seen CONTRIBUTING.md and the style guide?

[MQ-06]

@spageektti let me review the mistakes and get back to you!