Add contribution guidelines for Bazel documentation

[davidzchen]

Due the the fact that some of the Bazel documentation (such as the User Manual, Test Encyclopedia, most of the Skylark docs, etc.) are also published internally, and due to some annoying differences between the internal documentation framework and Jekyll (namely .md links rather than .html), it would be a good idea to provide some documentation in the Contribution Guidelines around this.

In particular, perhaps we should clarify which kinds of documentation changes should be avoided, such as changing .md links to .html for docs that are also published internally.

Some other ideas that might be useful:

• Documentation linter for catching these kinds of changes.
• Tests for validating the docs generated by //site:jekyll-tree (such as verifying that the jekyll-tree.sh script correctly converted all .md links to .html.
• Adding a design doc describing how the external documentation pipeline works (from Build Encyclopedia and Skylark Library docgen to packaging the Jekyll tree).

[steren]

To start simple, let's at least explain which version of Jekyll is needed, and that serve-docs.sh must be used to run the docs.

[jin]

Marking this as P2 as the build is getting more complex, and we should have documentation around this.

[meisterT]

What's the status of this?

[jin]

This should be on the Contribution section on bazel-website. I'll take an action item to make public our doc development instructions.

Transferring this to the bazelbuild/bazel-website repo.