Docs-978 fully branched PGD 5.6

[djw-m]

This is a future release branch - specific instructions must be followed when merging, see "Merging this branch" section, below.
This branch shall not be merged until PGD 5.6 is released!

Please use this branch only for changes specific to version 5.6 -
changes and corrections that apply to 5 should be made in short-lived
branches that can be merged into develop and released promptly.

Working on this branch

Other people may be editing and altering this branch. Be sure to pull latest before
beginning work - and be aware that this branch may be rebased on a regular basis to
incorporate changes from the previous version. We recommend using git pull --rebase or
setting pull.rebase to true in git config to avoid mistakes,
e.g. run git config pull.rebase true on your local clone.

This branch will live for months, but try to merge and delete your branches within hours or days.

For changes that need review or feedback, create a branch based on this branch and
request feedback on a PR before merging your changes into this branch.

Please use descriptive messages and comments for your commits and PRs - this allows
other people working on this PR to see at a glance what is changing.

Good: "updated version to 5.6 in index.mdx" / "correct misspelled option in CLI reference"

Bad: "updated index.mdx" / "fixed typos"

Incorporating 5 updates

To bring in applicable changes that have been made to PGD 5, rebase
this branch against develop and force-push. Be sure to review changes for areas
that should be updated for 5.6. When resolving conflicts, defer to 5
except when changes in this branch are necessary to support 5.6.

Merging this branch

When PGD 5.6 is released, follow this procedure to complete the separation of
documenation for 5 and 5.6:

1. Rebase one last time (see above)
2. Bring the 5 subdirectory back into the source tree:

   git checkout develop product_docs/docs/pgd/5/
   git commit -m "restore PGD 5 docs"
   git push
   

3. Immediately merge this branch into develop.
4. On a new branch off of develop:
   • Rewrite all /pgd/latest/... links in product_docs/docs/pgd/5/**/*.mdx to /pgd/5/...
   • Fix any redirect collisions that show up during a build (these will be redirects of the form /bdr/... that appear in both 5 and 5.6 trees)

[github-actions]

🚀 Deployed on https://deploy-preview-6017--edb-docs-staging.netlify.app

[josh-heyer]

Heads up: I've slightly altered how reference index generation is invoked, to allow for multiple versions of PGD (or, future, anything else) to make use of this.

• behavior is triggered by presence of a file named index.mdx.src. Any directory containing such a file will (eventually) get passed to refbuilder.js
• refbuilder dependencies are initialized on process startup (right after printing the docs banner). This remains a bit ugly (should happen outside of build entirely), but at least not being invoked every createPages pass (this was the slowest part of running refbuilder previously)
• any changes to input files will still potentially trigger multiple rebuilds in develop; they should be somewhat faster though.
• refbuilder will not be invoked in CI (github actions) builds. This was only necessary in situations where files were missed anyway, and only led to issues.

[josh-heyer]

⚠Rebased! Pull latest before continuing work. See updated description (above) for details on how this branch should be handled before and during the PGD 5.6 release. Refer to https://github.com/EnterpriseDB/docs/blob/develop/docs-for-docs/agreements/product-version-fork.md for details.