Various doc build improvements

[maximlt]

Fixes #289
Fixes #207

Also:

• Build the site directly with sphinx, no longer need nbsite build ...
• Build the site without using the NotebookDirective from nbsite
• Update nbsite (still needed to share CSS/templates and some config with other HoloViz sites) to its latest version
• Built projects all moved to the /doc/gallery subfolder, to be able to generate a toctree for projects that have multiple files and having them available in the left side bar (5f94e65)
• Redirect from the dummy homepage to the Gallery page
• Notebook headers generated from another custom extension, not to have everything in the gallery extension
• Fix Google Analytics setting
• Fix rendering $ and Latex math expressions
• Fix displaying images referenced from <img> HTML tags, using the html_image myst extension
• Make the docs workflow a little faster
• Don't include notebook sources in the built site (very heavy, not useful)

[github-actions]

Your changes were successfully integrated in the dev site, make sure to review
the pages of the projects you touched before merging this PR: https://holoviz-dev.github.io/examples/.
You can also download an archive of the site from the workflow summary page which comes in handy
when your dev site built was overriden by another PR (we have a single dev site!).

[github-actions]

Your changes were successfully integrated in the dev site, make sure to review
the pages of the projects you touched before merging this PR: https://holoviz-dev.github.io/examples/.
You can also download an archive of the site from the workflow summary page which comes in handy
when your dev site built was overriden by another PR (we have a single dev site!).

[github-actions]

Your changes were successfully integrated in the dev site, make sure to review
the pages of the projects you touched before merging this PR: https://holoviz-dev.github.io/examples/.
You can also download an archive of the site from the workflow summary page which comes in handy
when your dev site built was overriden by another PR (we have a single dev site!).

[github-actions]

Your changes were successfully integrated in the dev site, make sure to review
the pages of the projects you touched before merging this PR: https://holoviz-dev.github.io/examples/.
You can also download an archive of the site from the workflow summary page which comes in handy
when your dev site built was overriden by another PR (we have a single dev site!).

[github-actions]

Your changes were successfully integrated in the dev site, make sure to review
the pages of the projects you touched before merging this PR: https://holoviz-dev.github.io/examples/.
You can also download an archive of the site from the workflow summary page which comes in handy
when your dev site built was overriden by another PR (we have a single dev site!).

[github-actions]

Your changes were successfully integrated in the dev site, make sure to review
the pages of the projects you touched before merging this PR: https://holoviz-dev.github.io/examples/.
You can also download an archive of the site from the workflow summary page which comes in handy
when your dev site built was overriden by another PR (we have a single dev site!).