Align markdown docs with OpenAPI specfication

[oej]

• Remove duplicate text
• Move text that's ended up in the wrong doc

[taleodor]

Some inconsistencies in this PR:

• The date → createdDate rename on the collection hits the schema only. Both spec examples still say date, and the collection markdown's JSON example does too, while its field list now says createdDate. Openapi require changes #264 makes the identical rename, so the two PRs conflict on the same hunk.
• The moved release documentation uses distributionType throughout its prose and JSON examples, while the new "key attributes" list correctly says distributionId, which is the schema property. The examples lack the required distributionId, so they fail the schema the PR aligns with. The artifact section of the collection doc has the same split.
• The pre-release text was moved into the product-release doc verbatim from the component doc, so the product-release page now explains pre-releases in terms of "creating the Component with a new UUID". The release doc gets a different, divergent description of the same flag.
• Spec examples switch to tei://, but the component doc's new sentence still says "TEI URNs", and the product-release markdown example still uses a third form, tei:vendor:product@2.24.3.
• The date-time description now claims RFC 9557, while the pattern on the next line only admits YYYY-MM-DDTHH:MM:SSZ. The description and the regex disagree.
• Moved checksum examples use SHA_256 and SHA_512 with underscores. The enum is hyphenated, so those examples fail validation. Pre-existing, but the PR copies them forward as the aligned version.
• The discovery example ends with an encoded newline, %0A, which is an invalid value regardless of the URN-versus-URL decision.

Overlap with #262

This PR rewrites the artifact-format url / signatureUrl descriptions in the spec and the matching mediaType / url / signatureUrl bullets in tea-collection.md, which are exactly the lines #262 changes (mediaType required; url and signatureUrl always external and optional, with the server's download endpoints used when absent). Here url is left unmarked next to signatureUrl (optional), which reads as required, so it's a semantic disagreement as well as a merge conflict. Suggest rebasing this on #262 once it lands and taking those bullets from there, so the docs end up consistent with the download endpoints.

[oej]

Fixed some comments, waiting for #262 for another review