docs: bump cairosvg from 2.4.2 to 2.5.1 in /docs

Bumps [cairosvg](https://github.com/Kozea/CairoSVG) from 2.4.2 to 2.5.1.
- [Release notes](https://github.com/Kozea/CairoSVG/releases)
- [Changelog](https://github.com/Kozea/CairoSVG/blob/master/NEWS.rst)
- [Commits](https://github.com/Kozea/CairoSVG/compare/2.4.2...2.5.1)

Signed-off-by: dependabot[bot] <support@github.com>

Closes https://github.com/espressif/esp-idf/pull/6365

.gitlab-ci.yml

@@ -63,7 +63,7 @@ variables:
   CI_AUTO_TEST_SCRIPT_REPO_BRANCH: "ci/v3.1"
 
   # Versioned esp-idf-doc env image to use for all document building jobs
-  ESP_IDF_DOC_ENV_IMAGE: "$CI_DOCKER_REGISTRY/esp-idf-doc-env:v7"
+  ESP_IDF_DOC_ENV_IMAGE: "$CI_DOCKER_REGISTRY/esp-idf-doc-env:v8"
 
 .setup_tools_unless_target_test: &setup_tools_unless_target_test |
   if [[ -n "$IDF_DONT_USE_MIRRORS" ]]; then

docs/requirements.txt

@@ -4,7 +4,7 @@
 # matplotlib is currently required only by the script generate_chart.py
 matplotlib==3.3.1 ; python_version>="3"
 matplotlib==2.0.1 ; python_version=="2.7"
-cairosvg==2.4.2 # required by sphinxcontrib-svg2pdfconverter[CairoSVG]
+cairosvg==2.5.1 # required by sphinxcontrib-svg2pdfconverter[CairoSVG]
 sphinx==2.3.1
 breathe==4.14.1
 sphinx-copybutton==0.3.0