warnings/problems by building docs

[ReimarBauer]

docs$: make html show few problems

• WARNING: Invalid configuration value found: 'language = None'. Update your configuration to a valid language code. Falling back to 'en' (English).
• /docs/development.rst:205: WARNING: Title underline too short.
• gallery/plot_examples/plot_example_LS.rst:16: WARNING: image file not readable: gallery/plots/Linear_LS_AT-.png
• docs/installation.rst:2: WARNING: Duplicate explicit target name: "here"
• docs/installation.rst:2: WARNING: Duplicate explicit target name: "anaconda".
• docs/mscolab.rst:49: WARNING: Title underline too short.
• docs/mscolab.rst:49: WARNING: Title underline too short.
• docs/mscolab.rst:58: WARNING: Title underline too short.
• docs/mscolab.rst:58: WARNING: Title underline too short.
• docs/mscolab.rst:84: WARNING: Title underline too short.
• docs/mscolab.rst:84: WARNING: Title underline too short.
• docs/mscolab.rst:91: WARNING: Title underline too short.
• docs/mscolab.rst:91: WARNING: Title underline too short.
• docs/mscolab.rst:108: WARNING: Title underline too short.
• docs/mscolab.rst:108: WARNING: Title underline too short.
• docs/mswms.rst:346: CRITICAL: Title level inconsistent:
• docs/mswms.rst:377: CRITICAL: Title level inconsistent:
• docs/mswms.rst:405: CRITICAL: Title level inconsistent:
• docs/usage.rst:225: CRITICAL: Title level inconsistent:
• docs/usage.rst:237: CRITICAL: Title level inconsistent:
• docs/usage.rst:251: CRITICAL: Title level inconsistent:
• docs/development.rst:157: WARNING: Failed to create a cross reference. A title or caption not found: 'demodata'
• docs/mswms.rst:363: WARNING: Failed to create a cross reference. A title or caption not found: 'demodata'