chore: add changeset file

Signed-off-by: Camila Belo <camilaibs@gmail.com>

.changeset/techdocs-swans-check.md

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+---
+'@backstage/plugin-techdocs': patch
+---
+
+Packages a set of tweaks to the TechDocs addons rendering process:
+
+- Prevents displaying sidebars until page styles are loaded and the sidebar position is updated;
+- Prevents new sidebar locations from being created every time the reader page is rendered if these locations already exist;
+- Centers the styles loaded event to avoid having multiple locations setting the opacity style in Shadow Dom causing the screen to flash multiple times.

.changeset/techdocs-wolves-carry.md

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+---
+'@backstage/plugin-techdocs-react': patch
+---
+
+Creates a `TechDocsShadowDom` component that takes a tree of elements and an `onAppend` handler:
+
+- Calls the `onAppend` handler when appending the element tree to the shadow root;
+- Also dispatches an event when styles are loaded to let transformers know that the computed styles are ready to be consumed.