Fix inclusion of docs spec draft in docs build

[florimondmanca]

What does this PR do?

Cleanup for #7792

• Fix undefined links
• Add docs specs to nav, with a clear "warning: this is WIP" admonition.

Motivation

This is making the docs/build job on master and PRs fail currently:

INFO    -  The following pages exist in the docs directory, but are not included in the "nav" configuration:
  - meta/docs-specs.md 
WARNING -  mkdocstrings.plugin: meta/docs-specs.md: Could not find cross-reference target '[doc-spec-producer]' 
WARNING -  mkdocstrings.plugin: meta/docs-specs.md: Could not find cross-reference target '[doc-spec-templates]' 

Additional Notes

#7792 was opened in October 2020, and master has not been merged into it since then before merging. But meanwhile we merged #8152 which enabled --strict build mode that would surface the issues seen on master now that #7792 has been merged.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached