Fix section heading link fragments which start with a digit

[ScatteredComet]

• Closes Incorrect link fragments #11016

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• Does using -, ., / within a custom anchor break them? If so then some of my changes will need amending [Edit: they do, and have been removed]
• When the convention _doc_<page_slug>_<title_slug>: led to non-unique anchors (e.g. in upgrading_to_godot_4.6.rst & docs_changelog.rst where headings are used twice) I used _doc_<page_slug>_<section_slug>_<subsection_slug>:
• I did not check if all pre-existing section anchors fit these 2 conventions
• binary_serialization_api.rst headings contain lots of colons: what should we do? [Edit: they are fine as is]

I also think it would be a good idea to add something to the contributing docs and/or unit tests saying that you need to provide a custom anchor if you define a title which starts with a number, and define a convention.

Notes on this PR

I used ripgrep with the commands rg -U '^[0-9].*\n-' and rg -U '^[0-9].*\n~' to search for these, which corresponds to searching for [line with a number at the start] + [line break] + [~ or -].

I also saw a lot of page anchors which dont align to

The reference _doc_insert_your_title_here and the title should match.

from the "writing the manual" page, but I'll make another PR for that as its a large diff.

[ScatteredComet]

From the CI, it looks like including those special characters break the links; I'll remove them. Would be nice to know exactly what characters are allowed though if we're going to define a convention.

[ScatteredComet]

I tried to look for some specification for valid anchors but didn't manage to find anything, so I just changed all special characters to underscores.

[Calinou]

I also saw a lot of page anchors which dont align to

The reference _doc_insert_your_title_here and the title should match.

from the "writing the manual" page, but I'll make another PR for that as its a large diff.

Changing page anchors will break translations, so I would recommend against this.

[ScatteredComet]

binary_serialization_api.rst's heading links are actually fine as is e.g.

https://docs.godotengine.org/en/stable/tutorials/io/binary_serialization_api.html#float

so I don't think there's anything else that needs to be added to this PR

[ScatteredComet]

This is a different solution than the contributing docs used e.g. here

godotengine/godot-contributing-docs#75 (comment) suggests that this PR might not work