Meta issue for custom data.

Will try to move html.experimental.customData and css.experimental.customData to html.customData and css.customData.

• HTML attribute/attribute-value completion documentation #63955
• Associate custom data schemes with file suffixes #67742
• Handle non file:// workspaces for custom html/css data contributions  #66307
• markdown descriptions vscode-html-languageservice#57
• If attribute is named "data", an additional attribute called "data-" is added. Bug? vscode-html-languageservice#56
• Move experimental settings / contributions for custom data into stable #78068
• Write docs for using custom data vscode-docs#2919