Fix broken link

[shendiaomo]

Fix #1053 partly.
Each link on the bottom of https://sqlflow.org/sqlflow/doc/quick_start/ points to the original .md files, which leads a 404 error.
This commit changes absolute links to relative ones, and the next commit will modify sql-machine-learning.github.io to convert relative links to Markdown files to their rendered equivalents.

[tonyyang-svail]

@shendiaomo Thank you for submitting this PR.

The original absolute path makes perfect sense in the sqlflow repo. And we shouldn't change sqlflow repo to fix link error in sql-machine-learning.github.io repo.

To fix the link, we may need to change some configuration file in the sql-machine-learning.github.io.

Also, I put up a discussion on general strategy in updating the Homepage at sql-machine-learning/sql-machine-learning.github.io#91 (comment)

[shendiaomo]

@shendiaomo Thank you for submitting this PR.

The original absolute path makes perfect sense in the sqlflow repo. And we shouldn't change sqlflow repo to fix link error in sql-machine-learning.github.io repo.

Relative links work perfectly in github repos too, see https://github.blog/2013-01-31-relative-links-in-markup-files/. So this commit has no effect on readers of doc/quick_start.md, and it has only a little effect on maintainers of the documents (moving quick_start.md to another directory other than doc/ requires modification, but renaming doc/ needs no modification. The opposite happens on the original absolute links.)

To fix the link, we may need to change some configuration file in the sql-machine-learning.github.io.

I've investegated a bit, relative links seems to be the best and only solution to this problem, see https://nicolas-van.github.io/easy-markdown-to-github-pages/ and https://github.blog/2016-12-05-relative-links-for-github-pages/
Of course it's better if you have a better solution.

Also, I put up a discussion on general strategy in updating the Homepage at sql-machine-learning/sql-machine-learning.github.io#91 (comment)

[tonyyang-svail]

I've investigated a bit, relative links seem to be the best and only solution to this problem.

Great. Thanks for the solution. :)

[tonyyang-svail]

LGTM++

[typhoonzero]

@shendiaomo is there a following up PR to update sqlflow.org web site as https://nicolas-van.github.io/easy-markdown-to-github-pages/ describes?