Improve dom_id uniqueness in guides

[p8]

All headers in a guide get a unique dom_id to make anchor links work.
If a header is already present we would prefix it (and it's duplicate) with the dom_id of the parent node.
This would raise an error for headers without parent nodes:

../rails/guides/rails_guides/markdown.rb:47:in `dom_id': undefined method `[]' for nil:NilClass (NoMethodError)

          dom_id = "#{nodes[-2][:id]}-#{dom_id}"

This commit simplifies the dom_id uniqueness by only prepending the
parent node if it exists. This can still result in duplicates at the
same level, but for these we already show a warning:

*** DUPLICATE ID: "some_id", please make sure that there are no headings with the same name at the same level.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[carlosantoniodasilva]

Thanks! Does it make it more difficult to target those anchors in internal links within the guides (in case we want to point to them manually for instance -- not common but possible), because they'll get the node index now?

I mean, previously we could link by hand knowing they are prefixed by parent node, now it's index base so not as easy to do it.

[p8]

Yes, I'll change it a bit. We can leave the parent node dom_id as those should be uniq anyway.

[p8]

@carlosantoniodasilva I've made the changes.
For top level headings it just uses the dom id of the heading.

[carlosantoniodasilva]

Looks and works great, thanks for the fix! Just had one minor suggestion for consideration, not a blocker.

(test failure is unrelated)

[p8]

Thanks @carlosantoniodasilva I've made the change.

[carlosantoniodasilva]

Perfect, thanks!